I went through Watcher's Keep with a party of level 11-13 or so characters (I hadn't read much then about ToB or WK on the messageboard, so didn't really know what to expect) .. The first couple of levels were OK, but when I got to the demon level, it started to get NASTY.. Those tieflings and elemental-summoning glabrezu defeated me so many times I had to resort to sneaking by them.
And as for the final keep guardians (the beholder, marilith, drow etc).. Apart from Melissan (who I still haven't managed to defeat) this surely must be the toughest battle in all of BG2. The Rod of Resurrection was very handy, but several times I had to escape through the portal, raise my dead characters, rest, then return. And that was with planting about 25 assorted thief traps, skull traps, and glyphs around the place they came out, and summoning assorted elementals, pit fiends etc.
Needless to say I didn't fight Demogorgon at the end
