How do I create an outline collider for a tilemap, instead of having a collider around each tile?

I've added a Tilemap Collider 2D to the tilemap object, which ends up adding a collider to each of the tiles. But since these tiles specify the inside of a walkable room, I need a way of inverting the collider so the tiles themselves don't have colliders, but their outside does.

 

You must tick "Used by composite" in the Tilemap Collider 2D... and add the "Composite Collider 2D" afterwards.

 

Thank you! This helped a lot. Exactly what I was looking for.