Nice board, but I don't see how it would be superior to the original HeroQuest board for a "Defend-the-something" type quest. Indeed, for that type of quest idea, the HQ board is actually suited better, with a big center room (to set up the defense) and all corridors leading to that center.
So I would suggest to open a new thread regarding the dragon quest board, it's possibilities and it's technical implementation and leave this thread to the defense of the crystal/prince Magnus/some person.

edit:
especially the bit which obviously must be used as prison cells.
You just gave me an idea for a custom tile.