Udayagiri fort is built on an isolated hillock with massive granite blocks with a lot of perfection. The tombs of the Dutch Admiral Eustachius De Lannoy who was a Dutch Admiral and his wife and son are found in the ruined fort's chapel. Therefore, this fort was once named as De Lennoy's Fort or Dillanai Kottai. The tombstone largely lies within the strong walls of the chapel which is in a ruined state. On the other hand, the inscriptions on these tombstones are carved in Latin and Tamil languages. The official of the department of archaeology recently found underground secret tunnels within the fort premises. The building structures in the fort complex are mostly in ruined conditions. With the visible foundation building structures, the visitors can guess about the structure in the bygone years. The fortified walls of the forts height largely varies as per the geography of the rocky terrain. There are four gate ways to this fort in all the direction. The fort premises had residential quarters, chambers for soldiers, arms storeroom and other common buildings.
In the 17th century, Udayagiri Fort was originally built by the local ruler because of the strategic importance of this spot in the isolated hillock and its elevation of 79 m or 260 feet high. In the 18th century, Maharaja Marthanda Varma of Travancore or Venad rebuilt this fort during 1741-44 at the same spot under the supervision of a Flemish naval commander named Eustachius De Lannoy of the Dutch East India Company. Eustachius De Lannoy, later become the Chief of the Travancore Army. The fort complex is spread around 36 ha or 90 acres. An old foundry is located in the fort was largely used for casting different types of local guns.
The warriors of Tippu sultan who wagged war with the princely state of Travancore were captured and imprisoned plus confined in this fort for a long time. Under Colonel Leger of the East India Company's Army marched in 1810 into the princely state of Travancore through the pass at Aramboly to largely quell a rebellion under the able leadership of Vely Thambi Dalava. In the 19th century, the English East India Company's troops were permanently stationed. A huge foundry for the large scale manufacture of cannonballs, mortars and guns were also built within the fort premises under the direct supervision of the General. The Tamil Nadu forest department has converted the fort complex and its surrounding area as a bio-diversity park. The Archaeological Department of India has given the status of protected archaeological site status to De Lannoy's tomb.
Udayagiri Fort is easily accessible on the Thiruvananthapuram-Nagercoil National highway. It is just a few kilometer away from the National Highway. This particular fort is most sought after fort in the southern portion of India. Most of the domestic and foreign tourists who visit Kanyakumari in Tamil Nadu and Thirvanthapuram in Kerala never give a miss to this particular tourist destination.