Isometric stuff is a lot asked around the forum so here is one good method of doing that.
Includes projecting position from regular SquareMap to IsoMap and IsoMap to SquareMap.
Add wall with left mouse button (From isomap to squaremap).
There is also example how to use 2 Tilemap objects to draw isometric map. It is not perfect but, works.
Here is good info about the method used