From what we’ve seen, most visitors to Greenwich scurry up the hill to see the Royal Observatory and the prime meridian. At the observatory a line constructed in the pavement marks the position of the Greenwich Meridian, the consensus line of longitude 0°. Countless pictures have been snapped of tourists straddling the prime meridian line.
There’s no particular reason why the prime meridian had to run through the Greenwich Observatory. Unlike the equator, the choice of the position on the globe of the line of longitude 0° is arbitrary. The international prime meridian could have been anywhere. In 1884 the countries at the International Meridian Conference might have as easily selected Rome, Kyoto, Washington D.C., or Mecca as the agreed upon prime meridian. In fact the French for many years ignored the consensus of 25 nations who chose the Greenwich Meridian and instead insisted on using Paris meridian for their maps. Undoubtedly fixing the Prime Meridian line to Greenwich qualifies as a form of cartographical imperialism, a small sin on the scale of world history.
There is more to Greenwich than the Prime Meridian. Indeed, UNESCO has anointed Maritime Greenwich and not just the Royal Observatory as a listed World Heritage site.
Down the hill from the observatory, the Old Royal Naval College is a sprawling complex of buildings along the Thames. Designed in part by Sir Christopher Wren, the college receives particular mention in the UNESCO citation. While much of the college is off limits to casual visitors, there are several buildings that are open and can be visited.
Two good places to see at the Naval College are the Chapel and the nearby Painted Hall. The Old Royal Naval College claims, rather non-definitively, that the impressive ceiling of the Painted Hall is “probably the largest painting in Europe”. It is certainly without doubt most likely probably the largest painting I’ve ever seen.