Construction of the defensive walls was started by the Spanish colonial government in the late 16th century to protect the city from foreign invasions. The 0.67-square-kilometre (0.26 sq mi) walled city was originally located along the shores of the Manila Bay, south of the entrance to Pasig River. Guarding the old city is Fort Santiago, its citadel located at the mouth of the river. Land reclamations during the early 20th-century subsequently obscured the walls and fort from the bay.