Forts are a testament to the great past attached to the place, and these forts in Kerala tell stories through their walls.
In the early days, Kerala was a part of ancient Tamil Nadu. Thirukural and Thiruvalluvar mention the importance of wartime forts and their role in defense. The works of Sangam shed light on the construction of the fort and many other things. The details of the foreign travelers Pliny and Ptolemy, as well as in the Periplus of the Erithraean Sea by the unknown author, provide clues about the ancient cities and the commercial centers of Kerala with protective walls. Later Northern stories give a detailed account of the forts of Kerala. In front of the forts there were seven streams and a strong wall with broken pieces of glass. The gates of the fort were fortified with iron, copper and silver nails. It also mentions a watchman guarding the top of a seven-level tower where he observes the surrounding area with a Chinese tube telescope and fires arrows at his enemies.
Major cities, shopping centers, and ports had forts in the early days. Vizhinjam, the early capital of the Aay dynasty, preserved with fortified walls in the 7th and 8th centuries, appears to be the first to have existed forts in Kerala. Ancient forts can be classified based on the condition and materials used in construction. They are Vada (Mud forts), Kulti (Water forts), Stone circle, and Kottai (Land forts). Depending on the ownership, the Kerala forts can be divided into two groups: European forts and local forts. The major European forts in Kerala were the Portuguese, Dutch, French and English. The forts in Kerala were built by the Zamorin of Calicut, Kunjali Marikkar, the Mysore powers, Dharmaraja, Sakthan Tampuran, Ikkeri Nayaks and the rulers of Travancore.
The Europeans built forts in strategic places were to control trade. Therefore, these forts often function as warehouses and sometimes serve as administrative centers. It was the Portuguese who first discovered a new sea route to India, and the Portuguese navigator Vasco da Gama arrived at Calicut in the year 1498 as merchant. The Dutch were the first Protestant country to establish trade relations with Kerala. The history of Dutch in Kerala begins with the capture of Cochin from the hand of Portuguese in 1663.Unlike the Portuguese, they never tried to establish a Dutch empire in India or take over the land around their factories, except the Chettuva Fort, the only fort constructed by the Dutch in Kerala.

Anjengo (Anchuthengu) Fort

Anjengo Fort is located at Anchuthengu near Chirayinkeezh in Thiruvananthapuram district. This was the first fort constructed by the British to establishing trade with Travancore. It was constructed in 1685. This square fort has four bastions and has a flight of stairs at each corner of the fort. The rampart walls are over 15 ft high and the walls are made of laterite blocks cemented with lime mortar. You can also see some remnants of the past such as the remains of an old wooden flag in the north-west bastion. A few tombstones can also be seen outside the fort wall. The fort was ceded to the Political Resident at Trivandrum in 1810. It became part of Malabar and later in 1950 it was transferred back to the Travancore-Cochin state. It is believed that a there is a secret passage to the sea at the south west corner of the fort. The Central Archeology Department has declared this fort as a protected monument.

Bandadka Fort

After the decline of the Vijayanagara Kingdom in the 17th century, Nayakas, the Governors of the provinces started to establish kingdoms of their own. The Nayakas of Ikkeri were the most powerful in south-eastern Karnataka. Viswappa Nayaka and his successors built several forts. Among them the fort at Bandadka is a prominent one. Bandadka is situated 43 km from Kasaragod.

Bakel Fort

Located 26 km south of Kasaragode town, Bakel Fort is the largest fort of Kerala spreading over 40 acres (160,000 m2). Bakal Fort was built by Shivappa Nayaka of Ikkeri dynasty in 1650 AD. Unlike other forts, Bakal fort does not have any palaces or mansions. It is exclusively constructed for defence purpose. The fort's zigzag entrance and surrounding trenches reveal its defensive strategy. Holes on the outer walls are designed to defend the fort effectively from naval attacks. The upper holes meant for aiming at the farthest targets; lower holes below for striking as enemy nearer and the lowest holes to attacking enemy closest to the fort. Other important features are the water-tank, magazine and the flight of steps leading to an observation tower built by Tipu Sultan. The fort is surrounded by a beach that offers a beautiful view of the Arabian Sea.

Chaliyam Fort

Chaliyam fort was constructed by the Portuguese in the year 1530. The Raja of Vettattnad, who was in constant rivalry wih he zamorin of Kozhikode, enabled the Portuguese to erect a fort at Chalium at the mouth of the Beypore River. Chalium was a strategic site, for it was only 10 km south of Kozhikkode. Raja of Chaliyam or Parappanad also helped the Portuguese. The Fort Chaliyam was eventually besieged, captured and destroyed by the Kozhikode forces in 1571. Remnants of Chaliyam fort can be seen on the beach where Beypore River meets the Arabian Sea.

Chendamangalam Fort

Chendamangalam Fort is situated in the historic village of Chendamangalam near Ernakulam, in Kerala. The history of the Chendamangalam village is related to Paliam, an ancient Nair family from Chendamangalam. Paliam was the biggest joint Hindu family until 1952. Chendamangalam Fort was the lodging place of the Paliath Acchan, the Prime Minister of Maharaja of Kochi. Paliath Acchan played a great role in the history of Kerala, with the advent of the Portuguese. This is a Hillock fort. It is one of the oldest fort palaces built in the early 14th century. It is also an example of the architectural splendor of Kerala. This fort has witnessed all colonial aggression in the history of the past 500 years. A church, a temple, a mosque, and a synagogue is there in the vicinity of Chendamangalam Fort as symbols of cultural harmony existing in Kerala..

Chettuva Fort

The Chettuva Fort is the only fort constructed by the Dutch in Kerala. It was constructed in the year 1714. Located on the banks of the Chettuva River, the Fort h is in a shambles now. It is also known as Willing Fort and for some it is Tipu Sultan's fort as it was briefly occupied by Tipu Sultan. Only the ruins of the fort remain here and the remnants of the rampart and other dilapidated structures here and there. During flood a portion of the fort was washed away. Tipu Sultan who conquered Kochi also demolished the fort partially. The State Archeological Department took over the remnants and conserved as a protected monument.

Cranganore Fort (Kottappuram Fort)

Crangannoor (Kodungallur) also known as Kottappuram Fort was built by the Portuguese in 1523. It was captured and destroyed by the Dutch in 1663. It was located in a strategic position, on the mouth of the river Periyar. The proximity of river mouth and port at Kodungallur made the fort and its premises a town. The fort played a significant role in many wars between the Zamorin and the rulers of Kochi. In 1662 the Dutch fleet had made an attempt to conquer this fort from the Portuguese and they defeated the Portuguese in 1663 in which fort got damaged. After taking over the fort, the Dutch reduced it to the minimum and used it as an outhouse to guard their trade ships. The remnants of the original fort envisage that it was of 18 feet in thickness and made of laterite. Consequent on the aggression of Tipu Sultan on the Malabar Coast, Ramavarma (Dharmaraja) of erstwhile Travancore) (1758-1798) purchased the fort along with Pallippuram forts on 31 July 1789. In 1909, the Department of Archaeology of Travancore erected a memorial pillar inside the fort and decided to preserve the fort as public property as it was in a ruined state at that time.

Ettikulam Fort (Ezhimala Fort)

The Ettikulam Fort or Ezhimala Fort is located inside The Naval Academy Compound, at Ezhimala in Kannurwas built by the Portuguese and subsequently held by the French and the English. Ezhimala may be originated from the word Azhimala which means 'the mount near the sea'. It was the royal seat of Powerful kingdom which comprised the whole of the present districts from Wayanad to Kasaragod. The fort consists of three hexagonal prismatic structures one of which has been completely damaged and the side walls of the fort are of various in length and in ruin. Ezhimala was the capital of the ancient Mushika kings and Kolathiri Rajas.

Feroke Fort

It is believed that Tippu Sulthan had established the fort at Feroke on a hill on the bank of the Chaliyar River. The site of this fort at Paramukku, popularly known as Kottasthalam also referred to in revenue records. Remnants of the fort are declared as protected monument in 1991. There is a rock-cut cave belonged to the megalithic age in the same compound.

Kannur Fort (St.Angelo Fort)

Kannur fort is also known as St.Angelo Fort. It is located 3 km from Kannur town. The fort is an important historical monument and a popular tourist attraction. The fort was constructed in 1505 by the Portugese. The Dutch captured the fort from the Portuguese in 1663. They renovated the fort and built the bastions. The Dutch sold the fort to King Ali Raja of Arakkal in 1772. In 1790 the British seized it and used it as their military station 1947. The fort is in the Cannanore (Kannur) Cantonment area. It is preserved as a protected monument under the Archaeological Survey of India.

Kumbala Fort (Arikady Fort)

Kumbala fort, also known as Arikady Fort was built by Ikeri Hiriya Venkadappa Nayak in the year 1608. The fort is located at a distance of 2 km from Kumbla on the National Highway on a lateritic hillock.Excavations uncovered a large hall likely to be meant for congregation and large quantities of roof tiles, local pottery, Chinese pottery, lead bullets and the mould used to make them. The fort is a protected monument.

Padmanabhapuram Fort

Padmanabhpuram was once the capital of the erstwhile Travancore kingdom. This town is surrounded by a fort with an area of 187 acres. The ancient capital of Travancore might be constructed before AD 1601. The palace with an area of seven acres is situated in the very centre on the Padmanabhapuram Fort, amidst hills, dales and rivers. The palace which is situated in Kanyakumari District is under the control of the Archaeological Department of Kerala. The fort has its origin even before the seventh century of Malabasr Era. Before 1600 A.D. the fort was in mud. It took seven years for the completion of its work. The fort gained a definite power and structure under Maharaja Marthanda Varma who reconstructed the old fort into a granite fort in 1745A.D.

Palakkad Fort

0 The Palakkad Fort is located in the heart of Palakkad town. It was built by Hyder Ali from Mysore, the father of Tipu Sultan in 1766 AD and was lost to the East India Company in 1790 AD. The fort was later rebuilt by the British. The fort offers beautiful views of the city of Palakkad. The fort is well preserved by the Archaeological Survey of India.

Pallipuram Fort, Kochi

The Pallippuram fort was constructed in 1503 by Portugese. It is located at Pallippuram, on the northern part of the Vypin Island in the Ernakulam district. There is an open space inside the fort which allows secure passage to the cellar. According to local legends, this cellar is a tunnel which leads to the Cheraman Masjid in Kodungallur. The Pallippuram Fort was captured by the Dutch in the year 1661, following which it was sold off to the Kingdom of Travancore in 1789. It is a protected monument of the Kerala State Department of Archaeology.

Powel Fort

Viswappa Nayaka and his successors of Ikkeri dynasy built several forts in Kasaragod at Bandadka, Kundamkuzhi, Powel (Povval), Kumbala (Arikkadi), Kasaragod, Chandragiri, Bakel and Hosdurg. Powel Fort is the the oldest in this category. It is a square shaped fort with eight round bastions, ramparts with parapets, trench on all sides and with only one entrance. The fort is built with laterite boulders using sand and clay as binding material. There is no watchtower associated with this fort. There exist holes in the ramparts for placing guns. There were many buildings inside the fort. The Department of Archaeology has taken over the fort and declared as a protected monument.

Thalassery Fort

The Thalassery Fort was built in 1703 by the East India The fort is now under the control of the Archaeological Survey of India. Close to the fort is the St. Rosary Church and Brennen cemetery. The castle is square in shape with massive artistically carved walls and doors. The fort has gigantic walls, massive, intricately carved doors and secret tunnels leading to the sea. The entrance of the fort, set on a 10 meter tall massive wall, is one of its unique features. The fort was constructed with a mixture of lime, jiggery and white of the egg. It is said that the conspiracy of Arthur Wellesly to capture Pazhassi Raja took place at the underground chamber of the fort.

Thangasserry Fort

Named as St.Thomas Fort, the Thangassery Fort at Kollam was constructed in 1519 by the Portuguese. The fort had three towers and four bastions. In 1658, the Dutch conquered the fort. Although the Portuguese regained hold of some of the areas of the fort, it was recaptured by the Dutch in 1661. In 1795, the Thangassery fort was surrendered to the British by the Dutch. What now remain are its remnants with an arched gateway and a fort wall.

Thiruvananhapuram Fort

The Thiruvananhapuram Fort is located in the heart of Thiruvananthapuram. The fort was built to guard the capital city of the erstwhile Travancore kingdom and Sri Padmanabha Swamy Temple. The Thiruvananthapuram Fort dates back to the 14th century.. The Sree Padmanabhaswamy Temple and its premises were known as 'Mathilakam', which means 'the inside of a fortified area'. There remain small temples, priest’s quarters, other religious buildings and a number of palaces. It was King Marthanda Varma who decided to have a strong fortification instead of the old mud fort. Thus the construction of the existing fort was started in 1747. It was completed during the reign of Karthika Thirunal Dharmaraja in 1787. The fort, built as per the design of vastuvidya (traditional architecture style of Kerala) expert Thycaud Vishnuthrathan Namboothiri, is 11,320 feet in length of which 5796 ft is of granite blocks, 2445 ft of laterite plastered with lime mortar and 2919 ft is of mud. The average height of the fort is 15ft. the mud portion of the fort does not exist now. There are four major entrances to the fort namely East Fort, West Fort, North fort and South Fort besides small entrances at Sreevaraham, Sreekandeswaram and Vettimuricha Kotta. The State Archaeology Department took over the structure in 1985.

References:
Kerala Charitram (Kerala History) by A. Sreedhara Menon.
Kerala State Archaeology Department.
Malabar Manual by William Logan.
Malayalam Historical Novel "Rama Raja Bahudur" authored by C. V. Raman Pillai.
Malayalam Historical Novel "Rama Raja Bahudur" authored by C. V. Raman Pillai.
Place histories of Kerala - Thrissur District.
Place histories of Kerala.
Thiruvithamkur Charitram (Travancore History) by P. Sankunni Menon.
Valath, V.,V., K., Keralathile sthalacharithrangal - Thrissur Jilla(in Malayalam),[3] Kerala Sahitya academy 1981. 2. Kerala Charitradharakal (Historical Documents) by Naduvattam Gopalakrishnan, Valath,V.V.K mentions in a local Daily, Mathrubhumi on 1978 July 6 about a cave being found when a village road was being excavated near krishnankotta and subsequently being destroyed. This he mentions was part of the structure.
Wikipedia.