Drag a unit out of the pen and drop it on the catapult. That is the only way anyone goes anywhere.
Hold the field to wind, drag to aim, release to fling. Wind into the red and the rope snaps.
Land them inside the walls and they storm the banner. Land them short and they just mill about outside, confused, forever. There is no forming up, no bracing, no plan.
Your archers will shoot your own men. Constantly. This is not a bug and it cannot be turned off.