When you exit the Tube station for the Tower of London, be sure to notice the crumbling stone walls from that date back to Roman Londinium.
If interested, look into getting special tickets for the "Ceremony of the Keys". This requires advanced special tickets that are sold out months in advance. The ceremony occurs after the Tower is closed for normal tours. The special tickets allow admittance for the ceremony for a few hundred lucky people on the castle grounds.
-Paul