Reached the end, destroyed all the towers, and was fighting the dragon. Got its health down to about half and then had to quit. I did not die, just quit the game to continue later.

When I continued again today the dragon's health was immediately back up fully. I've been fighting the dragon for the past couple of days, and now that I think of it, I think this has happened before as well.

Do damage to the dragon, after taking out the towers, log out and quit. Then later on continue the game and the dragon's health is fully restored!