Flying siege?

[Arryn]

I just ran into something frustrating:

The Pangea AI in my current game left it's home fortress undefended (inside and outside). So, I say to myself, this looks like a very ripe target to go hit with my solitary flying-10 Vastness (w/ Gift of Reason). Alas, despite there being no defenders whatsoever (except the ones manning the missile throwers on the walls), I still have to break down the door with a flying unit. Argggh!

Besides the likely answer of "play balance", is there any reason why an army of all flyers needs to bother with breaking down the front door first, just to get into an empty castle?