shape, following the contours of Manila Bay and the curvature of the Pasig River. The Muralla walls covered an area of of land, surrounded by thick stones and high walls that rise to . The walls stretched to an estimated 3-5 kilometers in length. An inner moat (foso) surrounds the perimeter of the wall and an outer moat (contrafoso) surrounds the walls that face the city. Defense structures Several bulwarks (baluarte), ravelins (ravellin) and redoubts (reductos) are strategically located along the massive walls of Intramuros following the design of medieval fortifications. The seven bastions (clockwise, from Fort Santiago) are the Bastions of Tenerias, Aduana, San Gabriel, San Lorenzo, San Andres, San Diego, and Plano. The bastions were constructed at different periods of time, the reason for the differences in style. The oldest bastion is the Bastion de San Diego, which was built in 1587. The fortifications of Intramuros comprises several parts, the front facing the sea and the river, which were less elaborate and complex, and the three-sided land front with its corresponding bastions. Fort Santiago was built at the northwest tip where the sea and river converge, and this functioned as a citadel. Fort Santiago has significantly served as military headquarters of Spanish, British, United States and Japan during different eras throughout the Philippine history. In Fort Santiago, there are bastions on each corner of the triangular fort. The Baluarte de Santa Bárbara faces the bay and Pasig River; Baluarte de San Miguel, faces the bay; and the Medio Baluarte de San Francisco, which faces the Pasig River. Gates Before the American Era, entrance to the city was through eight gates or Puertas namely (clockwise, from Fort Santiago) Puerta Almacenes, Puerta de la Aduana, Puerta de Santo Domingo, Puerta Isabel II, Puerta del Parian, Puerta Real, Puerta Sta. Lucia, and Puerta del Postigo. Three of the gates were destroyed. Two of them, the Almacenes Gate and the Santo Domingo/Customs Gate, were destroyed by the American engineers when they open up the northern part of the walls to the wharves. The Banderas Gate was destroyed during an earthquake and was never rebuilt. Formerly, drawbridges were raised and the city was closed and under sentinels from 11:00 pm till 4:00 am. It continued so until 1852, when, in consequence of the earthquake of that year, it was decreed that the gates should thenceforth remain open night and day. Existing gates Destroyed