After realising that 17a is only really a small part of the theme I have made progress. The NW is now all that is left but it is bereft of answers. Is 4d thematic? I have no idea from my crossers what is going on.
rosencrantz
12a is an anagram of three of the words, the last less its final letter. ‘For building’ is the anagrind.
13d is an insertion of a 4-letter word for a set of steps halfway through a 4-letter word meaning confined.