Why do battles get harder in Strongholds? Can I reset the difficulty?

The more battles you win, the stronger the enemies get.

You can reset the difficulty back to battle 1, which is always the entry point for each Stronghold run. You can do this at any time by tapping the reset button in any Stronghold screen. The cost will increase for each reset and will be reset to 100 diamonds with the regular Stronghold reset. A coundown above the button indicates the time for that.