Co-op is limited to the Sandbox/level editor mode. One player acts as the host and can invite friends on Steam to join their server, or provide their IP address so players can join directly. Within the Sandbox, players can work together to make structures and then build different constructs (like cars, catapults, and more) to destroy them, or maybe you want to work together building a fort and work to defend it against an A.I. army. As a building game, the only limitation is your imagination.
Besiege is a physics based building game in which you construct medieval siege engines and lay waste to immense fortresses and peaceful hamlets. Build a machine which can crush windmills, wipe out battalions of brave soldiers and transport valuable resources, defending your creation against cannons, archers and whatever else the desperate enemies have at their disposal. Create a trundling behemoth, or take clumsily to the skies, and cause carnage in fully destructible environments. Ultimately, you must conquer every Kingdom by crippling their castles and killing their men and livestock, in as creative or clinical a manner as possible!