Oddly enough, there seem to be no doors to the tower. It appears as though it cannot be entered via land. At the top there is a conical shelter to keep the rain (and arrows) off anyone up there, and a trapdoor that leads down. Shyrath was right when she described it as small - though it is several levels high, it is not that wide, and certainly could not fit a Huge creature, even a Large one such as Myra would have difficulty getting through the trapdoor.