The Deadly Dungeon is the first A to Z Mystery book to take place outside of Green Lawn, and as a child, it was my second favorite book of the series. (My favorite was The Haunted Hotel.) I think it was the ghost/haunted castle aspect of the mystery that reeled me in the most; I loved spooky stories, though I didn't read them very often.
Rereading it as an adult, I have only one complaint: that the kids' parents are almost entirely absent from the books (in this case because the children are spending a week away from home). I've mentioned before in previous reviews of the series that this bugs me, mostly because there's such a strong disconnect between the old-fashioned "free range children" feel of Green Lawn, a town right out of the 1950's, and my personal experience growing up as a suburban 90's kid with extremely restrictive parents.
The other problem I had with The Bald Bandit and The Canary Caper doesn't reoccur in The Deadly Dungeon; this time, rather than having a mystery randomly foisted upon them for no explained reason, Dink, Josh, and Ruth Rose become entangled in the enigma of Moose Manor much more naturally and reasonably via their obvious curiosity and sheer proximity to the clues. I hope to see that continue in later books, given how little sense it makes for multiple random adults to seek these prepubescent children out as private investigators.
Ultimately, The Deadly Dungeon is another mystery from Rob Roy that should entertain (and perhaps even frighten a bit!) its target audience. I definitely recommend it to young mystery and/or ghost story fans.