Complex sentence: During World War Two a bomb shelter was built on top of the hills, as Hucknall was a good target with an RAF base, a Rolls Royce factory and the coal mines.
Split into simpler sentences: During World War Two, a bomb shelter was built on top of the hills. Nearby Hucknall was targeted by bombers as it had an RAF base, a Rolls Royce engineering facility and numerous coal mines.