as far as i know the game isn't 3d. it's all 2d as always except 3d-like objects. so as dpmasters said, ground tiles (and currispondent walkable/solid tiles) can be edited as always. in addition, a pseudo-3d-video software is installed so things like mountain and ponds are displayed as 3d objects, with some parts solid and some not (the higher one as always). if you disable the 3d part u can see a game as always, with a lot of grass/black ponds and nothing else. so they create 3d-displayed objects (dunno how it's codified and could be edited) and not as always tiles also for houses. to be clearer, a door-tile exists no more. the object "house" is an all 3d image, so it don't need regular tiles. the only tiles really existing in current game are mostly pavement's and indoor's ones, that as u can see, are as always. the 3d view part may be hard to hack, but not impossible. just i dunno how it's created, maybe it's smtg not too hard. a programmer may explain better (combination of various frames? smbd knows?)
i know is a bit confused, but understand this. in the middle of the screen u create a square of solid blocks. now, you can put there
-a 2d picture that u can see as 3d (the classic exagon)
..._________
/________/|
|.............||
|.............||
|.............||
|_______|/
and this is how it has always been done (in GB/GBC/GBA games)
-or a real 3d object, using a new graphic processor that can create "real" 3d. (ehe, can't paint this)
anyway, u have the pavement that's still a square of solid blocks that u can edit. but using 3d they've done a lot less number of tiles, coz the houses and mountains borders are 3d so don't need a 2d tile.
lol hope sbd will understand stg ^^''