This is a very interesting discussion. I think that it is possible that this could work. You wouldn't even need to be on the ground, but if you would want to be on the ground, you could always put a lashing to the ground, than make a stronger lashing in the direction you want to move. If you did add abrasion, though probably unnecessary, you wouldn't need any pike or spear-men, just let the weight of a hurtling mobile fortress smash into them. You'd probably need to add some sort of straps or something to make sure any Archers or control crews didn't go flying back. Imagine having these protect blocks of Infantrymen as you go up a hill! Or transporting troops across great distances in giant blocks. If this actually could work(which seems plausible), think of the tactical advantages!