The Defense Workshop is a building where defense tools are produced. Only one can be built in each castle/outpost. It has three levels, and at each level, different tools can be made to protect your castle from incoming attacks.
At level 2 you unlock Flaming arrows and Bulwark. At this point, you now also gain access to Dummies, which are defensive tools, that when your castle or outpost is spies upon fool enemy players into thinking that they are units. The three types of dummies are the Dummy melee soldiers, Dummy ranged soldiers, and Dummy Archers.
Disclaimer: The tools listed above are not all the tools in the defense category, they are simply the tools that can be produced by the Defense Workshop, for a list of (almost) all siege tools visit