Here is a brief description of how the creators describe besiege;
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!

This is an early access game on steam that costs £5 or $7, but, do not be put off by how it is not fully completed and polished, since it already gives you enough mechanics and parts to play with that you'll be satisfied and get your times worth already. As an example there are wheels, bombs, sawblades, spikes, poles, propellers, springs, suspensions, hinges, pistons and alot more stuff that can be keymapped to near any key on the keyboard. Not to mention the graphics already are quite gorgeous with cute animals, humans and bright red blood flying all over the place (especially when everything explodes)

Want to build a flying, firebomb catapulting, spiderlike grabbing, sawing, prodding, self destructing, walking battering ram? well if you can manage to fit all the parts together, then you sure can!
The developers at Spiderlings games are consistent at making new parts and updating the game regularly and are hoping to release V0.1 soon (it's at version 0.05 atm, showing how much content there is to come).