The first B tile is the tile that automatically fills all upper-layer-positions on the map, that's why it needs to be transparent and set to star.
Star basically means "use passability of lower map layer" (and also display non-transparent tiles above the player).
If you set the first B-Tile to o, then everything that has nothing from B-E placed on it will become passable, if the first B-Tile is x, then everything without a B-R-Tile on it will become impassable.
It's one of the quirks createdby the way maps were made, you have to know that - and the tips I mentioned in my post above from my tutorial also contain the other quirks of the engine...
This thread is being closed, due to being solved. If for some reason you would like this thread re-opened, please report this post and leave a message why. Thank you.