Siege Engines

Siege Engines can be either crafted "on the spot" (i.e. during sieges) or built elsewhere and then transported, depending on type of the engine and available carrying capacity. The ammunition mentioned so far is "Giant flaming rocks and maybe even giant flaming Dwarves", with the possibility of firing warriors/tanks on or over the keep walls also being considered!?

Developer Quotes
Mark Jacobs: Expect a wide range of siege equipment, and maybe some funny ones as well!?

Videos and Sources
Mark-A-Thon Part 9 (skip to 1:57 for siege engines)