Admiralty Arch is a large memorial to Queen Victoria that separates Trafalgar Square from The Mall. The Latin inscription across the top translates as “In the tenth year of King Edward VII, to Queen Victoria, from most grateful citizens, 1910.”
[Taken from my blog.] There's not a lot you can say about Admiralty Arch really. It's just a big arch. Well, that's actually not quite right -- it's three arches. Or three arches and two piddly little arches off to the side. But come to think of it, there are actually two interesting things that I can say about the arch. The first one is this: the middle arch is always closed. That is because the Queen is the only person who's allowed to go through it (a bit like Marble Arch). They open it up on State occasions so she can glide by in her golden coach. At all other times the traffic has to make do with the arch on either side.
The other interesting thing is that the strip along the top and the windows on either side all accommodate government offices. I could have sworn that there was a flat inside too, where John Prescott used to live as Deputy Prime Minister, but I can't find any info on that, so maybe that isn't true.