House construction

That's silly, it's like saying all you do in Mario is jump.

The server player could presumably move things around if needed and to facilitate their client's ascension/room clearing. His presence, however, would not have to be constant: this way, if the server player doesn't feel like playing the freaking Sims, he can go outside and kill stuff and explore while his client does his stuff.

I assume manual house building is more important early on when grist levels are low and player survivability is more effected by the tactical considerations afforded by house structure, and then lowers as the game progresses.

It'd be very plausible to have access to both, especially early on. Even then though, you have to build at least 5 or 10 floors before reaching the first gate, and there's no way players would enjoy going from killing monsters on their planet to playing Bob the Builder for 10 floors on their client's house, even if it's early in the game.

Perhaps you could make two modes for building: an 'automatic' mode and a 'manual' mode. The manual mode would be similar to the interface in Homestuck, with the standard controls. The automatic mode would randomly copy-paste existing rooms onto the top of the house, while making sure that every room is accessible, and that the client can proceed upwards or downwards quickly.