3-4 times I have had my island attacked and have been defeated. Each time, the % destruction has been under 20%. How is that a defeat?
It has to be your Pirate Hall being destroyed. Same rules as Clash of Clans: 1 star for main hall, 1 star for 50%, and the last star for everything else.
Yes, they must be taking out your Pirate Hall, as soon as you lose your PH the attacker automatically gets a 1 star win. I've always left my PH outside my walls, I've never had a base design with my PH inside, it's great for getting a 12 hour shield when people snipe my PH. My current base is great for keeping out the average attacks, they usually attack from the PH side & then get crushed once it falls.