I would have thought this was a Ninjago set until I saw the minifigs...And even so, switch out the brown elements for light bluish gray and it could serve as an expansion for 2505

These guys don't take themselves too seriously. First of all, you've got Sweet Clyde up on the battlements, laughing derisively, but then they can't even bother to hang their shields straight.

I always liked that horse helmet piece.

@MCLegoboy said:
"Shouldn't that apostrophe be on the other side of the S? Is there a singular Dragon, the title given to the ruler of the Dragon Knights, or is this faction known as the Dragons? If it's the latter, then it should be the Dragons' Prison."

Dragon is actually the name of the rat. It's his prison.

Was this the inspiration for one of those 2-in-1 sets from The Lego Movie? 70806

I can't help but feel like something is missing here...

Not to be confused with 7947. Or 7949.

…for a given degree of ‘prison’, sure

This. Set.

Got it for christmas but ended up selling most of my Kingdoms sets because they were very fig heavy and the builds were so puny. I'm not an army builder and seeing them together looked cluttered on my shelves and I wanted to make some space.

That was a huge mistake.

This was the WORST set out of all of them to get again after I sold mine originally! It was originally twenty euro and I eventually was lucky to get one for roughly 65 euro in a combination deal, which I had to pick up personally a large distance away, and with the barding clips broken.
Oh, and some of the jumper plates were also brittle brown, and I had to buy a whole Space Police fig I already had just to be able to get a replacement for the kettle helmeted lion knight's rare head.

And after all that, it's still not even that great of a set because the horse evidently took a lot of the budget from the build. I'm super happy to own it again and I appreciate it a lot more now for what it is... but man. Never again!

Oh, and I'm not going to get a replacement for that barding.

Anyway, this set comes with a fully decked out dragon knight. The visor and barding and torso armor only appears in this set, as do the gates in this color. The decked out knight's head is rare too.
The rest is a simple flick-fire balista, the gate, a prison cell a mini catapult and an open room with a red gem. It looks nice but it's built on two 4x4 plates with a 2x6 in between so it's very shallow.

I wish I had bought more but : €20 was quite expensive at the time.

@Mr__Thrawn said:
"Was astonished to find out how expensive the one dragon knight is a few months ago"

Kingdoms had a final wave of 4 sets before it got axed.
This set + the other 3 were only sold for 7 month

Kingdoms hat some really cool sets. I got three of them over the years MISB via Bricklink, the two smallest plus 10223, my favourite of the line.
Reminds me of when I used to play Defender of the Crown on my C64 back in the day.

"Sergeant Johnston, where's our dragon?!"

"Well, sir - you do know what this set is called, yes?"

"Alright, fine, but - where's the rest of our prison?!"

"It was a big dragon, sir. Quite angry. When it broke out, it -broke out-."

The archer doesn't seem to be too concerned about the prisoner escaping and his knight being under attack.