I think these should be end-game enemies. Attracted to large cities, high level players, and gold, it's attack the bases and destroy the shield before destroying the base. Of course you can kill it, but that'd be extremely hard without alot of people, since their power would scale depending on the size of city, players within, and amount of valuables. Catapults and a new build called a Ballista could be made, specifically for fighting dragons, could be built on stone towers for range. Dragon's stats would scale drastically, and would spawn in mountains, and head towards strong people and areas, so nabs don't get burnt alive.
Just thought this would be cool cause Nordic mythology has dragons.