I put the symbol into the bowl, it disappeared. Some sort of activity happened with the large symbol of helm; it looked like tiny lightning. Then nothing. No door has opened. I've done all 4 towers(both floors in each). Help? : /

Nevermind. Found it. Its a stair way right below the alter. Basically visually think of it as the middle part of the set of stairs leading to the dais lowering themselves into stairs now going down. Move your mouse over that area and you'll come up with a staircase icon replace the normal mouse icon.