I also spent a lot of time today making a short helper utility for painting walkable tiles on a tileset. It doesn't take long and they are saved to disk ready to be used by the level builder. When I want to use different layouts or styles in future tilesets I can simply use the script to paint walkable tiles directly on to the tileset pieces.
The end result of this is I can make a basic walkable dictionary for the level as soon as it is generated, instead of as it was before where I had to wait for all the tile pieces to be placed in game first before I could get walkabilty data. This will have some great benefits including being able to test the map for a valid route through the dungeon.
Items such as furniture and doors will be added and walkability data will be updated to take them in to account when the level loads. You can always smash a table if it is blocking your path.