Strongholds are lairs or caves occupied by an enemy clan. You can start a siege on their Stronghold. The first battles are very easy, but the more battles you win before the Stronghold resets, the more difficult they get. However, rewards will get better, too!
If it's getting too hard and you don't want to wait, you can also choose to refresh selected Stronghold for a Diamond fee at any time. But you will start a fresh run with the easiest battle again.
Are you up for this challenge? Attack and see how far you can get!