D&D Campaign Case: Terrain (Dungeons & Dragons Accessories)

Is dry erase markers usable on the terrain tiles, or how do you go about placing boundaries and walls on the tiles?

What are the diameters of the three sizes of plastic discs?

Are the weighted creature discs actually magnetic? One reviewer mentioned this, but nothing else (including the product description) does.

Should I use dry erase or wet erase markers on the terrain tiles?