The ruins of a coastal fortress sit atop a jagged outcrop along the beaches of Wales; experts investigate and uncover a derelict 19th-century British fortification that housed a series of occupants, including an exotic, private zoo. Stories: 1) St Catherine's Fort, Pembrokeshire, Wales; 2) Josip Broz Tito Political School, Kumrovec, Croatia; 3) Hackensack Water Works, Van Buskirk Island, New Jersey; 4) Calakmul, Campeche, Mexico