These Sea Forts helped keep a look out for incoming raiders. Three were located near Liverpool and four were in the Thames Estuary. They all had impressive artillery, like Lewis guns, Bofors, and 3.7 inch antiaircraft cannon. However, the ones close in the Thames also had radar. These antennae helped with tracking the low flying Luftwaffe planes dropping mines. With their information, the Royal Navy minesweepers were able to deal with this threat to shipping.