A sally port on Fort Canning Hill. Taken by User:Sengkang of ENglish.Wikipedia in Nov 2005.

"One of the at least three sally ports on Fort Canning Hill. Sally ports are small doors which lead in and out of forts. They are meant to help defenders enter or exit the fort undetected, and to be used in case of attack. The word "sally" means to make a sudden, vigorous exit. If an enemy is besieging a fort, the defenders can try to make a sudden attack through a sally port and surprise the enemy outside."

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