The entrances into the lower levels are hidden. Belcorra clearly planned to be able to hide parts of the gauntlight. She even murdered her own servants upon learning of the Roseguard attack in order to cover her tracks. She may even have deliberately gone up to the first floor to meet them just so that she could prevent the true nature of the Gauntlight in the lower levels from being discovered, even if she had to die to do it.
Why is there a "floor" on the level 1 "base" of the lighthouse itself, rather than just one giant open column. It seems like structurally there's no reason for it, but perhaps it is intentional that the lighthouse plausibly appears to be just a "normal" lighthouse with nothing underneath it.
