The Dragon is the weapon of last resort, to use if you see that you won’t be able to hold a Wall. Using it carries great danger, but usually positive effects will overshadow the negatives.
If you want to use Iron Dragon you will have to assign Clerk to it. First player will have to spend resources to cast the cannon, but will get to choose on which Wall Section it will be deployed. It is obvious, that Emperor will bestow great Honor on the one, who created such a great weapon. Then, all players can send their Clerk to shoot from the Iron Dragon. When player shoots, they draw 1 card from mini-deck, showing what effect the shot had. Effects vary from blowing huge hole in the Horde card, through just scaring them off for a turn, to cannon exploding and destroying current level of the wall.
If you do not like the random effect you can play that you can choose the effect - even exploding it sometimes, so other player cannot use it or you can build it (and earn Honor for doing so) yourself.

THE GREAT WALL - IRON DRAGON BLISTER - EXPANSION (ENGLISH) is categorised BOARDGAMES / ADVANCED GAMES / COOPERATIVE GAMES and sold by l'Imaginaire, a store and specialised website.