I really like this tool. One change I would love to see is more variety in the placement of elements. In particular, I'd like to be able to position the Citadel portion in the centre of the city, rather than abutting the outer wall.
This will be implemented eventually as a part of a greater plan (tweet) :) For me a castle surrounded by houses looks unnatural, at least it needs some open space around. But as I said, it will be possible to achieve this way or another.