So, when you use a dragon for military purposes, it's probably for the best to keep them close and keep them happy. Which means building them a place they can call home.
Now, housing a dragon is quite difficult, you have to proof the place against hissy fits when the dragon start flipping tables, you have to make sure the home is defensible if assassins come around, and most importantly, you have to make it comfy! A warm fireplace, a soft bed of the right size, etc...
So, your typical western dragon with six limbs in total. Size is generally where I have a breakdown but I say 2-2.5 meters at the highest point (where the wing's humerus is located) and 500 kg in weight, thanks to pneumatization. The wings span at around 12 meters and are the broad soaring type.
The dragon's scales provide some protection and the pneumatized layer between the skin and muscles makes laying on a hard floor more bearable, still, the dragon demands to have soft things they can lay on.
The total body length is 10 meters, half of which is the tail, their main weapon. Dragons have flexible necks and usually keep their head up, further adding to their effective height.
While dragons have a breath weapon, they never use it indoors.
Dragons have human intelligence. Though they can manipulate things with their wings and forelegs (they can opening a door) and with their mouth (like horses), they're rather gawky/clumsy.
This dragon doesn't really like humans but does like cooked and baked food, so he'll tolerate them.
Assuming late-medieval tech, what would a house for a dragon look like?