"Escape to Salvation" - "Magnificent Retreat" [Campaign 4, Mission 4]

I can't figure out where to place Zephyr's castle. I finished the game, and there's no castle, but the ability is there.

In the final mission, the castle cannot be summoned – in fact, it's not needed at all, because Zephyr receives the necessary reinforcements every week (thunderbirds), and friendly air elementals are waiting for him in two places on the map (around >400 in total).