Here is the option I used when making the flat map. These ones are the default ones when I opened it.
As you can see in the picture, I am floating on the door of the first floor,eveen the enviroment sprites and monsters.
Here's also my DATA.INI for reference.
[Data]
1=maps.grf
2=large.grf
3=palettes.grf
4=server.grf
5=rdata.grf
6=data.grf