Lewes is situated on the Greenwich Meridian, in a gap in the South Downs, cut through by the River Ouse. It is approximately seven miles north of Newhaven, and an equal distance north-east of Brighton. As of April 2010, it is part of the newly-created South Downs National Park.
This map is courtesy of Google, and includes the Streetview feature. Drop the little orange man on where you want to look at and you'll be amazed!