London, central to English and Roman importance even in the seventh century, was the site of Uther Pendragon's coronation after the death of his brother (cousin?) Aurelius Ambrosius.
The castle, of course, was a much later addition. This is how the London Tower looks presently.
Lodging, though expensive, is widely available in London. If you are planning to stay in a hostel during the summer months, (the peak travel season) make reservations well in advance.
Carter Lane Youth Hostel
36 Carter Lane EC4
071-236 4965
Hampstead Heath
4 Wellgarth Rd. NW11
081-458 9054
Holland House B&B
Holland Walk, Kensington W8
071-937 0748