The Haven hangs in space. A vast star city, devoid of life. Organic life, that is. 

From their high spire, looking out over silent streets and empty plazas, the Assemblers are waiting for the day when the humans arrive. Waiting. Waiting. Waiting.

When the TARDIS brings the Doctor and Lucie to the Haven, it seems like Assemblers' long wait might be over. Living beings! Without batteries! Protocol be praised!

Except - they're headed for the lower levels. They don't want to do that. That's where the Cannibalists live. And if the Cannibalists catch them - well, they won't be living beings much longer.