Jatopian's saying that it's probably not a good idea (or at least it's not the intended use) to use custom graphics to hide information from the player (as was much easier to do in earlier versions of DROD - e.g. with spiders, things hidden under tar).
I did point out that hidden monsters could be annoying when beta-testing this hold, but I think you decided to keep them.
Overall though, I wouldn't let this put people off playing the hold. There's a lot of good in it, and there's only really one room where hidden monsters are irritating, I'd say.
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