Over millennia, a great deal of human ingenuity and effort has been expended on producing defensive structures. These include forts, citadels, castlesCastle, moated settlementsMoated settlement, causewayed enclosures, trenchesTrenches, and frontier walls. Special attention is given to hill fortsHill fort, ringfortsRingfort, Nuragi of SardiniaNuragi, Sardinia, Ksour of North AfricaKsour of North Africa, causewayed enclosuresCausewayed enclosure in the UK, forts in the Roman EmpireRoman Empire, Crusader and colonial castles, moated settlementsMoated settlement in the UK and ThailandThailand, war trenchesTrenches, frontier walls, and concentration campsConcentration camp.