Tip on closing the great gate fast first you might need lighter and less item drop some items outside the gate. When you get inside you'll a siege which is a metal crawler. So go in either tower and there will be a crack the one on the right has a bigger gap so choose left jump over the gap save before doing that it takes a while to sucessfully jump when you made it the tower will right in front of you and you know the rest also after closing the siege crawler will be destoryed so walk to it and activate it will drop a sigil stone. P.S the sigil stone found at the great tower cant be used to enchant weapons, armour , items you'll need to give it to Martin Septim.