[Tuidjy]

> teleport your army on an enemy army if it's out in the open

He did not ask about "out in the open". The army in the castle will move even
if an enemy has laid siege by teleporting, as long as the orders to march out
were given on the same turn as the orders to teleport.

I am not guessing here. I know it for a fact, up to v3.10. I very much doubt
they changed it in the last patch.