You arrive somewhere else, as the TARDIS overrode the co-ordinates. The doctor says at UNIT HQ a hundred years before it is founded, which at the time is a priory converted into a mansion.
You discover an English mansion room is filled with Sarcophagi and Egyptological artifacts. The doctor says he wants to explore the mansion to find out what threw the TARDIS off. You make a few rooms before you find the main door from your wing to the main hall is locked.