For the time being, Orange uses a very simple map tile collision algorithm.  The player character is on a particular layer.  If the tiles on that layer are anything other than tile 0, they're considered solid.  What this means is that maps should generally have 3 layers:

• The lowest layer is mostly just the ground and objects the player can walk on top of.
• The middle layer is for walls, tables, counters, beds, bushes, whatever -- stuff that the player can't walk through.
• The upper layer is for things that the player walks underneath, such as roofs, the tops of trees, etc.

It's perfectly okay to have additional lower and upper layers.  If you want to have a layer of grass, then a layer of flowers that the player can walk on, then just make two layers below the middle layer.  If for whatever reason you need a layer above the upper layer, that's fine too.