if you want all black parts to be transparent (I mean totally black= color $000000, and totally transparent, no milky glass thingie) then you may set the masked texture flag. Since you don't load the texture manually you have to use the automatic filter, then after loading reset it to defaults:
cleartexturefilters
texturefilter "", 9 or 4
map=loadmesh("castle.3ds")
cleartexturefilters
texturefilter "",9 ; defaults
cleartexturefilters
texturefilter "", 9 or 4
map=loadmesh("castle.3ds")
cleartexturefilters
texturefilter "",9 ; defaults