When Karthika Thirunal was just four years old in 1728, his father died foiling an assassination attempt by the Kayamkulam Rajah when the prince and his parents traveled from Haripad to the realms of the Brahmin leader Vanjipuzha Thampuran in Budhanur. As he grew up, he actively participated in his uncle Maharajah Marthanda Varma 's military conquests and supported him in creating Travancore's modern state by annexing kingdom after kingdom to Venad's Cochin.
Karthika Thirunal Rama Varma succeeded to the Travancore musnud with the death of his uncle Marthanda Varma in 1758. Karthika Thirunal started his reign with the able services of the Dalawa Ayyapan Marthanda Pillai. Under the rule of his uncle, Travancore had acquired the reputation of being Kerala's most powerful state and, hence, many of the neighboring chiefs wished to execute friendship treaties with the Maharajah. At the lowest ebb were the fortunes of the Cochin royal family. Alone a very small portion of their original territories remained in the hands of Cochin Raja. Since 1755 AD, Calicut Kingdom's Zamorin (Samoothiri) was in control of Cochin's main parts.While Marthanda Varma had offered assistance against the Zamorin by signing an alliance treaty with Cochin in 1756, he did not take any measures to help. The bulk of Kochi's barons had sided with the enemy. The Netherlands were Cochin's permanent friends and protectors, but on their way to Cochin they withdrawn their contingents from the field when the Zamorin promised to send them 'Chetwai' which he had captured from them. Cochin's Raja invoked his nephew asking Rama Varma for prompt assistance.Naturally, the Cochin rulers' recollections of past shady dealings and breach of confidence raised fears in the Maharaja that his neighbor could not keep his promise when he felt his position secure. The same feeling may have stopped his brother, Marthanda Varma, from providing immediate assistance. Months passed by. Finally the Cochin Raja met Rama Varma in order to press his case. With religion's assent he strengthened his claims.
On 1761 AD the Raja of Cochin read the terms of their treaty and solemnly pledged to abide by its terms, in the presence of the Deity Sthanumoorthy of the Temple of Suchindrum near Cape Commorin, the ministers of the two kingdoms and the spiritual authorities were also present and the signed agreement was delivered to the representative of the Maharaja of Travancore. The Travancore Maharaja, according to the treaty, ordered his minister Dalawa Ayyappan Marthanda Pillai and his Dutch General D'Lannoy to move north at the head of a large army to liberate Cochin 's kingdom from the clutches of the Zamorin of Calicut state.The account of this campaign is best given in the words of late Mr. C. Achyutha Menon, a Cochin native whose familiarity with the Cochin government records, to which he had long been secretary, allowed him to speak with unassailable authority.
The Zamorin sent an messenger to Trivandrum, asking the Maharaja to hold his General's side. Just as his uncle Marthanda Varma had ordered him and Dalawa Ramayyan to withdraw from Cochin 's conquest when they were about to do so in 929 M.E., Rama Varma Maharaja now ordered his Dalawa no further advance. The Cochin Rajah sent his nephew to sign a treaty with Travancore in 1759 AD. A few months later, the Rajah himself and his minister Paliath Achan visited Travancore the same year and signed a treaty with Travancore and obtained assistance to free his territories from the domination of the Calicut Zamorin.The Zamorin 's offer to forgive him was fulfilled in 1763, and in the following year the Zamorin visited Padmanabhapuram, Venad 's capital, and concluded a peace treaty and reimbursed Travancore for the war expenses amounting to Rs 150,000. Upon pensioning off the ruling families, the little kingdoms of Parur and Alangad were also annexed to Travancore.
Marthanda Varma had intended to construct a line of fortifications on the northern frontier but his death the following year stopped anything major from happening. The experience gained during the war with the Zamorin persuaded the Dewan and the General of the need to continue and reinforce the lines that would not only secure Cochin to some degree from any potential attack by the Zamorin, but would also be useful to Travancore in the event of a Mysore invasion. Then, the scheme was carried on.Cochin's Raja was in complete agreement with the plan. The line of fortifications was taken from the sea near Vaipeen Island right up to the approximately 40 kilometer (25 mi) distance ghats. In addition to enabling the building of the several portions passing through Cochin territory, the Raja of Cochin decided to pay a portion of the costs.In fact the Dutch records stated that Maharajah Marthanda Varma intentionally refrained from an earlier occasion annexing Cochin in order to establish a buffer-state between his kingdom and Hyder Ali's Mysore. Subsequently, the famed Nedum Kotta or 'Travancore's Northern Defense Line' became a major obstacle in the attempt to conquer Travancore by Tipu Sultan; he named it 'the contemptible wall.'
Several territorial disputes occurred between Travancore and the Nawab of Arcot, which were eventually resolved, but the Maharajah had to part with large amounts of money and some territories mainly because it was forced by the officers of the British East India Company, as the Nawab was a familiar figure in Madras and his propensity to borrow continuously at extra hazardous interest rates enlisted t In return, the Maharaja acquired lands in Shencottah and the temple at Cape Comorin, which were not part of Travancore at the time but were wanted by the Maharajah to round off its dominions.
The maharajah died at the age of 74 on 17 February 1798, after a long, successful reign, save for Tipu Sultan 's invasion of the kingdom. He had preserved Travancore 's sovereignty and secured it from destruction by Mysore's superior forces. He retained the relationship his uncle Marthanda Varma had established with the British. Most specifically, because of the refuge he offered to the thousands of people fleeing from Malabar to avoid forced proselytization by Tipu Sultan, he came to be known as Dharma Raja. Certain subjects of Malabar were so happy that many families of kings and nobles remained back at Travancore.