For lovers of magnificent architecture, London has it all. There are ancient Roman walls, elderly edifices like the Tower of London and the Houses of Parliament and Big Ben, and big mega-modern monoliths like The Shard.   Take along your Rick Steves' guide book for information about all these wonderful buildings or ask your knowledgeable guide, who will likely know a few more tidbits of information.  Rick has two great London guide books, the full size guide and the Pocket London guide, which we chose because we could just pop it into our Rick Steve's Civita day bag and be on our way.