Just finished level 6- so far I am enjoying this. One suggestion I have so far: make the longbow the required weapon to kill the level 6 boss. After all, armor-piercing was one of the advantages of a longbow over a short bow in reality :-)
Edit:
one days later, and I am done. A very good quest. I will admit that I didn't get the magic key (I just watched James' video to see how to get it), but it definitely is just a trophy, as I think I only had one locked door left at the time I found the route, but not the trick to getting by the room that needs the "creative" thinking.
I liked the puzzle aspects- the rooms in dungeons 5 and 7 with the push-block paths were very interesting (although I did the one in level 7 before finding the clue room).
Several things were interesting, but got annoying after a little while. In particular the overload in the number of types of wizrobes (I think I counted 6 types, it may be 7) I think the variants were overused. The invisible enemies were done in a much better manner- they were common enough to be more than a novelty, but sparse enough to not be annoying- the only really tough room with them was guarding the red ring, and I can allow that- for 2x health, a challenge is called for.
One question I have is in dungeon 7, there is one room not on the map, but it isn't a hidden path. I think that this was probably an oversight, and the dungeon map should be corrected accordingly (the missing room breaks the symmetry of the map as well, which hurts it aesthetically.
As I said above, the longbow really should have had a purpose, and my suggestion stands, although another idea comes from my experience in level 9- either I was really unlucky, or the ghoma in this level required the silver arrow to hit (I can't believe I messed up THAT many times with the normal arrows, but it is possible). If that is the case, then perhaps making the longbow required here instead would be better, perhaps with a clue somewhere saying something like "some ghomas' armor needs a stronger bow to penetrate" which would clue you in to the need for an upgraded bow, if you missed it.
Overall another great map (I've hit some of the better ones late in the set). I give it a flat 4 out of 5, ranking it second out of the 11 5th quests I've done so far.