The Tower of London Castle has very secure entries (to make certain that visitors have a paid ticket).
Any other way in would require navigating a fenced moat and climbing a wall.

This photo was taken by a Kowa Super 66 medium format film camera with a KOWA 1:3.5/55 lens and Kowa L39•3C(UV) ø67 filter using Kodak Portra 160 film, the negative scanned by an Epson Perfection V600 and digitally rendered with Photoshop.