It is easy to take our most important historic monuments for granted, and there is a commonly-held view that we know everything about them. Archaeology, however, has the capacity to surprise us by revealing new, unexpected aspects of even the most familiar sites. So it was at the Tower of London, when Historic Royal Palaces established the Tower Environs Scheme in the mid 1990s to improve the area around the castle and provide it with a setting worthy of its status as a World Heritage Site. The idea of reflooding the moat was one of the most exciting proposals under consideration, and a major programme of archaeological and technical investigations was undertaken to assess its feasibility. This led in 1995 to the very unexpected discovery of top-quality stonework at the bottom of a small trench in the moat. The discovery was difficult to interpret, so a much larger excavation took place during 1996-7, revealing previously unknown parts of the Tower's 13th-century defences. This report provides a full description and analysis of the remarkable discoveries, representing a major advance in our understanding of Henry III's expansion of the castle. Many other interesting aspects of the moat's development and environment are also described, and there are extensive entries on artefacts such as a wickerwork fish trap - and the fish trapped inside it.