I just posted this in that thread:
If you have that tile set to X, it means you will hardly be able to walk anywhere. If you have it set to O, it means you'll be able to walk almost everywhere.
Your map is made up of layers. You have two bottom layers, and one top layer. You can't control which one you're drawing on. Each layer has a grid, the same width and height of the map, and each "cell" in the grid contains a tile id. If you haven't drawn on that cell on that layer, the tile id it contains is 0. This is the first tile in the B tab.
When the game checks for passability, it starts from the highest layer and works its way to the lowest layer, until it finds a tile that has passage settings. So if you haven't drawn on the tile, it will use tile id 0, the first tile in the B tab, and look at its passage settings. If it's X, it says no passage, and stops checking. If it's O, it says passage is okay from all directions, and stops. A tile that has its passage set to a star tells the program to ignore the passage of the tile on the current layer, and look at what's beneath.
That's why the top left B tile always has to be a star - it's the 'erase' tile, and the 'default' tile for when you haven't drawn anything, and it tells the program to look at what's on the layer below to find the passage setting.
