Darlan: Do they know about the Doorwardens yet?

Cylinder: No, I don't think so.  You tell them.

Righty-ho.  The Doorwardens were a last-minute addition, you see.  In all the early drafts we were going to have Daleks on the doors, but there were a few incidents of violence on set...

We lost two good crewmembers filming Episode 32.  A boom op and a focus puller.  Very regrettable.

Quite.  And there was a bit of a... I dunno, what would you call it, Cylinder?

A clash of ideologies, I suppose you could say?

Yeah, that sounds about right.  So we had to let them go, and come up with a replacement quickly.  We found the Doorwaden characters sitting around on a metaphorical back-burner looking sorry for themselves, waiting for their threequel film to be made.

One other note on the subject - the Doorwardens as you see them in the final cut are all played by Andy Serkis via motion-capture.