The Seige Workshop is one of the Military Buildings. 

In game quote:The workshop for siege weapons doesn't look like anything else in your empire. Perhaps it will be more appropriate to call it "machine-building factory". Here your loyal subjects, under the strict direction of the head imperial engineers, produce and put together your siege weaponry. The access to this super secret facility is quite restricted, of course, and to make it even more hidden it is placed somewhere in the deeps of the imperial forests. For this reason the construction of siege machinery is perceived by the usual folks more as magic than anything else. The truth is that only the prices of such machinery are truly magical.

-Builds Siege machines.-Base: 20 siege machines.