Trafalgar Square is one of the two main places that demonstrations end up in London, and I've probably been there for demonstrations more often than I have just wandering around London. On May Day 2000 I was stuck behind police lines there for several hours. It's a great rallying point, and a fine stop-off place for a passing tourist even now that they've starved out the pigeons. Otherwise, though, there's just not much call to go there unless it's New Year's Eve and you really love enormous crowds.
The Square is designed to be impressive, and it doesn't disappoint in this respect. The sheer size of the space, the enormous lion statues, and the massive public erection that is Nelson's Column, give it the air of an Important Place where Important Things are likely to happen, probably involving Important People with Important Ideas. All of this goes to explain why so many political rallies happen there - plus, it's only about ten minutes' walk from the Houses of Parliament, and the New Labour government passed special legislation a few years ago to make it illegal to demonstrate anywhere they might actually see you. Trafalgar Square is just about safe.
Besides the big lions and Mr. Nelson, there are four massive plinths around the square. Three of them hold brass statues of people who were doubtless Very Important a hundred and fifty years ago, two of them being Generals I've never heard of. The Fourth Plinth is far more interesting, having been empty for many years and seen a series of artworks occupying it since 1999. The best known work to have sat there so far was Mark Quinn's rather beautiful marble sculpture of 'Alison Lapper Pregnant', a naked, very pregnant and very obviously disabled woman, which some people found a little disturbing - it exposed us to something most of us don't often see. As Quinn says on the Fourth Plinth web site:
At first glance it would seem that there are few if any public sculptures of people with disabilities. However, a closer look reveals that Trafalgar Square is one of the few public spaces where one exists: Nelson on top of his column has lost an arm.