five;
[there is a scribbled mass of equations and a hastily-drawn diagram of the front of the castle]
I'm sure everyone has noticed the rather large... Heartless, I think they're called -- and, possibly, the sudden appearance of a keyhole in the main doors, yes? They seem to be extremely intent on getting at it, which, logic would suggest, would not be good for us at all. What we need is a way to protect it; something like the containment field generator that Romana built may work, if we build another and adjust it for the main doors.
It'll have to be done quickly -- there's an army heading towards the castle.
I don't suppose you ever happened to get a technical readout of Gallifrey's Transduction Barriers during your Presidency?