A rollinglog is a type of fortified unit consisting of a large rolling log. These logs weigh a lot, roll downhill quickly, and will crush attacking units to death.
They are a rather ineffective 'one-shot' weapon. Scout and Cavalryspam attacks can trigger and waste them for little return.
Construction time is affected by Construction Research and your Hero's Politics Skill. Construction Time = BaseTime*(0.9^Const)*(0.995^Poli)