Siege workshop is a building where siege tools can be produced. Only one siege workshop can be built in each Castle or Outpost. More siege tools are unlocked with each upgrade. You can build Resource Tools, or instantly receive other more effective tools for Rubies. For example, the variant of the Loot sack would be the Loot cart, and the Scaling ladder variant would be the Siege tower. The production is increased by 5% when you upgrade it.