Built to defend and protect Old Goa from potential invasions, Fort Aguada is the largest and the best-preserved Portuguese bastion in the state. It was built sometime between 1609 and 1612 to guard against the Marathas and the Dutch forces that wanted to establish their reign. Its strategic location limited the entry into River Mandovi and also to protected Old Goa from enemy attacks. Among the most fascinating highlights of the fort is a large cistern that could store over 20,00,000 gallons of water and a magnificent citadel.

A four-storey lighthouse, built in 1864, sits prettily on the premises, and is believed to be the oldest of its kind in Asia. During the initial days, the lighthouse emitted light once every seven minutes that was reduced to every 30 seconds in 1834. The lighthouse was ultimately abandoned in 1976. The fort has been named after a freshwater spring located inside it that once provided water to arriving ships. Though the fort has lost its earlier grandeur, some of the buildings are still intact and have been converted into a prison.  Tourists can reach the hilltop fort either from a 4-km-long road from Sinquerim Beach or a 2-km steep footpath. Aguada Fort is a fine example of Portuguese construction and engineering and is the best preserved Portuguese fort in India. It is said that so well built and fiercely armed was this fort that it never fell into enemy hands. It also boasts of a secret passage which was used in the times of war and emergency.