Mine's not art heavy (no new art needs to be created), it is "developer heavy", in that the developer has to decide what locations to mark, but it'd be very little effort (depending on your map editing tools), and take very little memory.

Yeah, it is a good approach.

Personally I prefer "non heavy". With my approach I just program a system that finds walls and ceilings and I don't have to mark out indoor areas at all.

Both have their advantages and disadvantages. Your disadvantage (and a minor one) is the developer marking out the rooms. Mine is a runtime hit of having to line trace periodically to find the ceiling.

The time I save in developer effort is paid back at runtime in a relatively expensive maths operation... :)

Sean and @Servant, these are good solutions but they are kind of art heavy

...not at all. If you design your tools well, it takes zero extra effort. In fact, you can even use braindigital's method to have the tool automatically number the indoor/building tiles; but then unlike his system, you have the freedom to override it when you need to (e.g., when you want an open-air tent that doesn't actually have walls to auto-scan for, or one of a bazillion other edge cases that the automatic system won't handle).

I think i will just mark all tiles with a unique ID when i place each building (Seans suggestion). Its easiest for me.

Good idea to darken the outside! There is two players however so i need to check what "none of the two players see".

Thanks for your input.

Erik