In this case, it was "Bailey's Tump" - a mound.
It's recently been restored as an historic site, as it was used during WWII as a searchlight and anti-aircraft site - as one of Sheffields 'outer defences', apparently they did actually shoot one German plane down.
According to the sign on the site, it's called Bailey's Tump because it was Bailey's field, and the tump was formed by the spoil left over from the construction of the Derwent Agueduct in 1900 - which carried water from the derwent dams to the city of Leicester.
If you google "Baileys Tump" you don't get much information, but you do get lots of video clips of it's official opening after 'restoration'.
Perhaps 'tump', in this case, comes from 'hump'?.