The Walls of Aquileia in The Fall of Rome in Age of Empires II: The Conquerors
Defensive structure is a type of building available in every game of the Age of Empires series. Rather than be used to train units or research technologies, they are used to defend their civilization from attacks, or warn it of attacks.
Generally speaking, there are three types of defensive structures: Walls, towers, and Castle-type buildings.
List of defensive structures 
Edit
Age of Empires Edit
All defensive structures are researched and upgraded from Granary:
Towers found in the campaign and the Scenario Editor:
Age of Empires II Edit
All defensive structures are researched and upgraded at the University.
Constructable Edit
- Outpost
- Watch Tower
- Guard Tower
- Keep
- Bombard Tower
- Palisade Wall
- Palisade Gate
- Stone Wall
- Fortified Wall
- Gate
- Castle
Unconstructable Edit
The unconstructable defensive structures can only be found in the Scenario Editor.
- Sea Wall
- Sea Gate
- Sea Tower
- Sea Fortification
- The Accursed Tower
- The Tower of Flies
- Barricade
- Poenari Castle
- Fortress
Age of Mythology Edit
All defensive structures (except for fully upgraded walls) can research technologies that upgrade themselves. Towers can also research technologies that improve building Line of Sight.
- Sentry Tower
- Watch Tower (Egyptians only free automatically upgrade Classical Age)
- Guard Tower (except Norse Heroic Age)
- Ballista Tower (Egyptians only Mythic Age)
- Wooden Wall
- Earthen Wall (Chinese only Archaic Age)
- Stone Wall (Classical Age)
- Fortified Wall (except Norse Heroic Age)
- Citadel Wall (Egyptians only Mythic Age)
- Great Wall (Chinese only Mythic Age)
- Bronze Wall (Atlantean Classical Age)
- Iron Wall (Atlantean Heroic Age)
- Orichalkos Wall (Atlantean Mythic Age)
- Mirror Tower (Atlantean only available through the worship of Helios, except Gaia)
Age of Empires III Edit
- Outpost (European civilizations only, except Russians)
- Blockhouse (Russians only, Chinese and Indian campaign with Russian Consulate ally)
- War Hut (Native American civilizations only)
- Nobles' Hut (Aztecs only)
- Castle (Asian civilizations only)
- Fort (European civilizations only, with Home City Card, Chinese and Indian campaign with Russian Consulate ally)
- Agra Fort (Indian Wonder)
- Trading Post (With Advanced Trading Post Home City Card)
- Dock (only against Ships when garrisoned by Fishing Boats)
- Town Center (only against ships except when garrisoned by villagers)
- Fixed Gun (Campaign and Scenario Editor only)
- Wall (All civlizations except Sioux)
