The walls and especially the gates of Old London were popular places for churches and chapels, as they were close to main thoroughfares where great crowds passed continually. The medieval walls, which generally were built on Roman foundations, began at the Tower on the banks of the Thames east of the City. They described a semi-circle around to the Thames west of the City where a fortification called Bayard’s Castle stood. There were seven gates: Aldgate, Bishopsgate, Moorgate, Cripplegate, Newgate, Aldersgate and Ludgate. There was also a gate at the northern approach to London Bridge. Today the physical walls with their gates and bastions have virtually disappeared. But churches remain on the sites of six of the gates, and another stands on the line of the old wall. All Hallows, London Wall, St Botolph, Aldersgate, St Botolph, Aldgate, St Botolph, Bishopsgate, St Giles. Cripplegate, St Martinwithin- Ludgate, St Sepulchre-without-Newgate.