"Ah, so it's a double deal sort of thing. Nice to know." The Doctor looks like he's about to go on more in that vein when he suddenly gestures at the big wall and the gate. "Rather big wall you have there. Trouble in the past?"

He doesn't think it'd do much good against an alien species capable of swarming planets and destroying entire worlds, but he keeps that to himself. It's still a rather nice, pretty wall.