Crac des Chevaliers - major Crusader site in the Middle East
Defence Line of Amsterdam
Vauban -quintessential site for European fortress building
Masada- Built for fortification against Mighty Romans
Dacian fortresses- Ancient Military
Suomenlinna (Also fits here)
Frontiers of the Roman Empire
Hill forts of Rajasthan
Great Wall of China - iconic and universally known
Agra - Maybe - not sure whether its main feature is its military function, or rather its palace function; also unsure how to rate it vs Rohtas fort or Red Fort.
Agra can be better representative of entire medieval South Asian History.
Lahore was capital for mere 13 years where as Rohtas was built during short life span of Sher Shah (and never has much impact in South Asian history). Red fort, Delhi was built by Shah Jahan and we can only see late Mughal Architecture (which are not that much beautiful).
On the other hand Agra was capital throughout Medieval era and example of whole range of architecture.