I think that the stoneworker should provide a bonus to walls and the door of dwarven castles, I really think that should be a way to consolidate their castles just like Gondor. Dwarves are renowned for their solid buildings, it would be nice to have some possibilities for the castle. I also find that the improvement that brings all structures in the fortress to level 3 is not very usefull, I think a defensive improvement would be more interesting, I suggest a defensive upgrade on the doors of dwarven castles using oil, it could be great and never seen in an other factions. If you have any ideas/suggestions please leave a comment.
(and sorry for my english).