Pendennis Castle (Cornish: Kastel Penndinas) is a Device Fort, or Henrician castle, on the west side of the estuary of the River Fal, near Falmouth in Cornwall. Together with St Mawes Castle its companion fort on the opposite east bank, it was built by King Henry VIII between 1539 and 1545 to guard the entrance to River and to defend Carrick Roads from the perceived French and Spanish threat of naval attack.