Hi Quisling
In the 100+ issues since I started subscribing, I have submitted entries on 2 occasions, so I am probably not in the running for most top answers!
Even if I did submit this one, it is not a prize one...
I feel I need to know the theme before tackling the cipher. I have tried standard rotation ciphers, and using 'quarters' as a keyword. Letter frequencies add to the confusion, with the enciphered 'e' being the most frequent! I feel it is most probably a 'keyword, then the rest of the alphabet' type of cipher, with the keyword being the theme. One of the NW quadrant's thematics seems to have a letter appearing 3 times, so is most probably the way in. However there are several possible entries, many of which could belong to a theme!
As to which of the clued answers is thematic, where does one start?!
It also seems unusual that all the thematic words have 6 letters. And I feel that 'quarters' must refer to something more than just the grid design...