[konidias]

Okay, you basically want to use a different type of tileset then the default. Here is all you have to do:

addtiledef image,levelstart,type;

replace image with the tileset name, replace levelstart with the name of your level, and replace type with 1 instead of 0.

so something like

addtiledef picso.png,testing,1;

edit: Yes, thak was right (I missed his post) You just replace 0 with 1 to get the nw tileset type. However, I wish Stefan would add a way to define what tiles do what. Of course it would have to be like a serverside option because otherwise people could just edit the tileset offline and walk through walls online.