Built from 1634 until 1765, for centuries the Castillo San Cristóbal has been standing guard at the eastern gate of Old San Juan. The imposing fortress was built to guard the city from enemies approaching by land, serving as a counterpart to Castillo San Felipe del Morro. The citadel covers over 27 acres and is the largest one made by the Spaniards in the New World. Fort San Cristóbal, as it is also known, defended the San Juan Bay from foreign attacks, such as the Battle of San Juan in 1797, perpetrated by Ralph Abercromby. The vast esplanade is a great place to explore around and enjoy the beautiful view of Old San Juan. This whole system, which is San Juan National Historic Site of defense, has been a UNESCO World Heritage Site since 1983.