Kerala was Invaded successfully only by Tipu Sultan and his father Haider Ali . I am giving a brief description of the early invasions in Kerala.
Canara forces invaded northern Malabar in 1732 at the invitation of the Arakkal. Under the command of Gopalaji, 30000 strong Canara soldiers, easily overran prince Kunhi Ambu’s (Cunhi Homo) forts in northern Kolathunad. By early 1734 the Canara soldiers captured Kudali and Dharmapatnam. By 1736, the Canara army was driven out of the whole of northern Malabar with assistance from the English East India Company. However, the Prince Regent incurred a huge debt with the Company factors at Tellichery as a result.
Hyder Ali first marched to present day Kerala in 1757 as per request of King of Palghat who was a long-time military foe of the Zamorin of nearby Kingdom of Calicut. Hyder Ali, who at that time was the Faujdar of Dindigul under Kingdom of Mysore, with a force of 2,500 horses and 7,500 men supported by Palghat troops, marched into southern Malabar. The army defeated the Calicut army and reached as far as Arabian Sea. The main intention of this movement to Malabar was to capture the vast treasuries of the rulers of Malabar. Malabar Coast was famous for its foreign spice trade from ancient times. Zamorin came to a treaty with Haider Ali, in which he was demanded to pay twelve hundred thousand rupees as the war reparations. However, the Zamorin technically deceived Hyder Ali after the return of the Mysore Army from Malabar.
The Calicut army failed because Hyder’s troops were organised, armed and trained in the most modern fashion whereas Calicut army, like rest of armies of kings of Malabar, relied on feudal levies. Zamorin eventually agreed to pay ₹1,200,000 as indemnity to Hyder Ali and so Hyder Ali withdrew. King of Calicut, despite the invasion, did not modernise his army – a neglect for which he paid nine years later.
When news of Hyder Ali’s conquest of Kingdom of Bednur reached Ali Rajah of Cannanore in 1763, he promptly requested Hyder to invade Kerala and help him deal with Zamorin of Calicut. This Muslim ruler of Cannanore, an old of rival of the neighbouring powerful Kolathiri, was an active ally of Mysore during the years of occupation.
Hyder Ali agreed and in 1766 he marched into Malabar through Mangalore with a force of 12,000 infantry, 4,000 cavalry and a park of field guns. During this time he was desperate for a port which opens to the Arabian sea, as his French allies were supposed to transfer weapons, ammunition and horses against the British. [, Mahe, a French controlled port, lay in the middle of Malabar. With his modern army, Hyder Ali easily defeated all the petty kingdoms on the Malabar, beginning with Kolathunad.
Ali Raja of Cannanore, a long rival of Kolathiri, seized and set fire to the palace of Kolathiri Raja. The latter escaped with his followers to the then-British settlement at Tellicherry. After the victory, Hyder Ali entered the Kingdom of Kottayam in present-day North Malabar and occupied it, with assistance from native Muslims, after some resistance by the Kottayam army.
The first serious resistance encountered by Hyder Ali’s army was in Kadathanad, followed by a series of atrocities against the natives.After the conquest of Kadathanad, Hyder marched towards Calicut, the headquarters of Zamorin. Zamorin decided set fire to the gunpowder store of his palace and thus committed self-immolation.Hyder Ali, with a large amount of money, marched south-east and moved towards Coimbatore through Palghat. Mysore appointed Ali Raja as military governor and Madanna (a former revenue officer) as civil governor of the newly acquired province of Malabar.
Shortly after Raza Ali, who was Hyder Ali’s lieutenant in command, returned to Coimbatore, Hindus hidden in the forests rebelled against the Mysore authorities. They, supported by the English East India Company, re-occupied forts and large portions of land in the monsoon season. Eralppad, second line successor to the throne at Calicut, continued his attacks against the Mysore forces from southern Malabar. Eventually, forced by continuous instability and rebellions, Hyder Ali agreed cede many parts of Malabar to local Hindu rulers (as age old customs existed in Malabar) as tributary states under the Kingdom of Mysore.
In 1767, the whole of Malabar again revolted Mysore’s army of 4,000 men, who were defeated by 2,000 Kottayam Nairs in Northern Malabar. All baggage, arms and ammunition of army was looted by the Nair rebels. Mysorean garrisons were trapped by Nair rebels who seized the countryside and ambushed Mysore convoys and communications with great success.
Now one must be having the idea that capturing and controlling Kerala was one of the toughest job in those areas . There are a number of reasons why Kerala was never conquered before Tipu Sultan by the Hindu Kings like Marathas and Mauryans .Mughals needed a solid footing near Kerala to stage an attack, which I guess they never had, as they never conquered Tamil Nadu or Karnataka fully. Only major invader Kerala had was Tipu Sultan who conquered most Kerala except Travancore. By the way Tipu with his Mysore empire was at an optimum place to launch attack on Kerala.
1.) Hindu Kingdoms: Most of the kings of Kerala were hindus and most of the population was Hindus who were living a very peaceful life unlike the rest of India who was reeling under Muslim rule.
2.) Geography of Kerala : Kerala is not just too far from central north India, it’s terrain is not easy for warfare. Mountains, rivers, forests and long rainy season made it hard to invade by land in older days. At least in Asoka’s days Kerala may not have had anything worth to invade.
In most cases, western ghats protected Kerala. Even very few South Indian kingdoms attacked Kerala.
3.) Strong Nayars: Kerala with its Kalari martial art culture gave a tough resistance to the invaders . The three major kingdoms of Kerala-Travancore (Thiruvithamcoor~Venad), Kochi and Nediyiruppu Swaroopam had valiant kings who were always able to protect their territory and their people to a very large extent from all sorts of trader turned invaders and expansionists whether they were the Slave Dynasty, Mughals, Portugese, Dutch, British and neighbours like Tipu. The Dutchs were defeated .Marthanda Varma defeated the Dutch army with the Nair Brigade in 1741 at the Battle of Colachel and captured the Dutch commander Captain Eustachius De Lannoy .Only when the french brother of Rothschild gave rockets to Tipu Sultan ( to blast temples whose vaults had enormous gold ) and Tipu Sultan used local Mappilas ( arab – muslim half breeds, almost like anglo Indians ) to spy and give covert support .