Castle, Fortress, and City upgrades

[PeterKnight]

I would like to suggest being able to upgrade castles and fortresses through 5 levels. Each level would require maxed copper and silver. After reaching maxed level 5, they would then be upgradable to fort or city vice linking 5 of each to form the next level.
Cities would be upgradable to level 25. Cities would also be able to earn you 1000 gold a week at level 25.