So I've done searching, and for the life of me I cannot find any topics or explanation as to how mapmakers are overcoming the built-in creep behavior. Currently if you make a square platform and put a creep colony on it, it will be edge to edge for the sides and top, but not the bottom. There will be a 1 square height that doesn't get covered by creep. Normally I'd just say that it's impossible, but other mapmakers are overcoming this issue!

I've attached screenshots of both the intended behavior from two sunken defence maps, and one from my failed attempts.

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Random guess: They are using a different type of black tile that is marked as buildable.

That kind of makes sense, because if I open the map with SCMDraft and save it, the terrain is no longer buildable. Whichever the case, it seems that I cannot use SCMDraft to even preserve these maps let alone edit/make my own with it. Something else is going on here.

Try placing the creep colony in high terrain and put low terrain around it. Or the other way around. Finally use a half walkable role that blocks walking below.

Edit: like the terrain sato uses in poker defense to allow archons to attack farther

I'm not sure what they could be doing that opening and saving the map would remove, but as SI said just lining the bottom edge with a buildable null tile (any null tile from a row of terrain that is buildable e.g. dirt, platform, etc.) works. This is most useful on space tileset, since the null will be indistinguishable from other tiles.

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Could you post that desert map please?

Well if only the MTXM layer was modified and not the TILE layer in an external program, then scmdraft obviously won't see those changes.

I don't think that creep will naturally traverse different terrain heights without existing creep up there, but they are technically "buildable" so it makes sense that it will go over slightly.