The game looks fine. I played a few times before I got the point what should I do first to win it. The idea is also very nice and only small tweaking can be done (but it is not necessary). For most of the time I had wood shortage and buildable terrain shortage.
1. When you start the game second time, You can't place the castle second time. To workaround it you have to close the game (complete exit) and reboot it. Then it is normal.
2. Removed stones are only removed. The spot is not buildable area. And the stones don't respawn (haven't noticed any). So I don't know why I can't place a building on that spot.
3. Once I have noticed that I had some workers and some empty buildings but the workers didn't go to the buildings. I don't know how to reproduce that.
1. Possibility to destroy buildings. If you accidentally place a building, the worker goes there and you can't restore the place and worker.
2. Savegame is also a good idea, but not necessary (since the game is quite a short). But if you consider make another "levels"/"missions", please consider savegames.
Hope you will upgrade the game! :)