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Trajan founded this as a colony for retired soldiers around AD 100 and the surveyors laid it out as a perfect grid, the decumanus running east to west and the cardo north to south, crossing at the forum, built for about 15,000 people. Then Rome left, the town declined after the fifth century, and by the eighth it was empty. Nobody built over it. The Sahara covered it instead, which is why what you walk through now is one of the most complete Roman street plans anywhere: the 12-metre arch of Trajan, a theatre seating 3,500 that still gets used, four bath houses, the Capitol, a Byzantine citadel, and a public library from the late third or fourth century whose inscription reads that hunting, bathing, playing and laughing is living. UNESCO listed it in 1982. Batna Province, about 35 km east of Batna.