Red Dragon Spawned next to Keeper - bug or misunderstanding?

[Cazier]

I'm new to the game, so maybe I'm misunderstanding. My first run just ended because a Red Dragon spawned next to my Keeper inside my dungeon.

I knew the dragon was coming - I'd seen the warning that I'd triggered it.

So, in preparation I extended the main entrance tunnel with doors, traps and a guard room etc.

From reading other posts, an unreachable Keeper can lead to this behaviour.
However, I had 2 entrances into my base (both 1 tile wide).

Does the Red Dragon have a teleport spell?
Or, is it so big that I need to make sure my bases have a 2 tile wide path through them to stop this?

I'm trying to figure out if this is a bug or something I could have prevented.
As it stands, it just seems like doors/traps/guards aren't very useful.