expression, godan verb (-su)
capture a fort; take a fortress
Used in military or historical contexts for capturing a fortified position. The verb 落とす here means 'to bring down' or 'to capture'.
See also: 落とす
敵軍はついに砦を落とした。
The enemy army finally captured the fort.
落とす alone means 'to drop' or 'to lose', but in this phrase it takes the meaning 'to capture'.
Compound of 砦 (fort) + を (object marker) + 落とす (to drop/capture). The use of 落とす for capturing a fortification is an extension of its meaning 'to bring down'.