Abyss
It's lonely in the deep
Given the number of dragon related posts may I suggest they use balista as their primary weapon?
Balistas are neat, but I'm not sure how many people would have dragons in mind when building defenses what with Drogon being the last living dragon known to the world (although it wouldn't be unfeasible seeing as they managed to come back after a 300 year absence).
My character won't be starting out with world domination in mind. Depending on his backstory, he's raising the dragon so that he can gain independence from his family and escape (after all, you need a crew for a ship, but single person can ride a dragon). His reasoning isn't perfect, and he'll have to learn to accept that running isn't an option and sometimes you have to pick a side or eventually someone will make the choice for you. I'd say he's more likely to have his loyalty swayed to a specific person, rather than a banner or crown.