The Armory is a building that provides military improvements for your base. It has upgrades in which will improve your army. The Armory itself costs 100 power, but the upgrades its has costs much more, ranging from 500 to 2000 power.

Note that once your Armory is destroyed, all your upgrades will be gone and you will have to research them again. So a good strategy is to keep them well protected from enemies.

Also, you can only build 1 Armory. It make senses because there's no point in researching something twice.
Name (of Upgrade) Cost Description
Power Armor 500 Increases soldier armor.
Booster Engines 600 Increases tank movement speed.
Panzer Cannons 1000 Adds cannons to your tanks, making them ranged units.
Cloaking Device 2000 This hides tanks from enemy towers, only spotters can see the tanks.

Adapted from

Support Generator GeneratorHouse House
Offensive Armory ArmoryBarracks Barracks
Defensive Wall WallSimple Turret Simple TurretSniper Turret Sniper Turret