Improvement request - rails
You already have most of the required features already in the game.
Add Rail Wheels and Iron or steel tracks and timbers for the ties...
Metal spikes would hold the rails to the ties.
It would be interesting to have a mining cart and it would be a blast to build your tracks down to your castle. You do have gravity implemented right?
Midevil roller coaster!
Hidden defensive catapult on rails...load it up, roll it out and fire. Roll it back in your cave and be safe while you reload.
Used vertically, it could be an elevator or if on a slant and you are a Willy Wonka fan, a Wonkavator.
As this is more a building game, it would give the creative players out there some interesting things to do for a more advanced game.
Perhaps it would justify higher capacity mining, refining and smelting tools for mass production?