Ancient drawbridge unearthed at the Tower of London . A drawbridge five feet below the surface , was discovered when the cause of a subsidence at the entrance to the Tower of London was investigated . It is thought likely that the excavations , which are to be continued , will provide valuable addition to the knowledge of the early histor of the Tower . The subsidence occured after recent rain in the small triangular green inside the palisade where the sentry stands . Then attempts to fill it in failed digging began . Large slabs of masonry were soon found and further excavations revealed chases for a counterpoise . Photo shows the chases unearthed at the Tower of London . 13 March 1936