St. Paul's is London's largest cathedral, built by Christopher Wren after the site's earlier cathedral was destroyed in the Great Fire of 1666. The structure is HUGE, with a magnificent dome. We climbed the 530 steps from the cathedral floor to the Golden Gallery atop the dome to get this view:

If you look carefully, you can see the Tower Bridge over Angela's shoulder and the terror in my eyes. We're 280 feet above terra firma at this point. I'm not letting go of that railing.

The official St. Paul's Cathedral web site is very well done, with interactive maps and 360° panoramas.