So you can see the rest of the dungeon, but there is no access to it anywhere? That is a rare but known bug with the dungeon generation.
There are a couple tricks you can use to get through walls in the main game, but they require either entering and exiting the BF or using the teleporter stones, neither of which are possible in this case. There is a way to get through, but requires a bit more technical expertise;
Teleporting anywhere (via hex editor)
