Six-fingered Claw

An old vigilance tower, maintained semi-regularly by hobgoblins (they can no longer access it on a regular basis as this is now in gnoll territory). The entrance to it is well-trapped & inconvenient to reach, so the gnolls haven't bothered inhabiting it; for them keeping the island's inhabitants in check doesn't require fancy towers. Unknown to the gnolls, however, is that this is an alternative entrance to the megadungeon below the island. If they knew that, they would, without a doubt, make the effort to climb all the way into the tower & post a regular patrol on it.