If you had just one day to visit Petra, besides the Treasury the highlight definitely not to be missed is Ad Deir, the Monastery. It equals the feelings of wonder and of witnessing eternity that are prompted by the sudden view of the Khazneh at the end of the Siq. But instead of emerging from the towering canyon walls, the Monastery is carved out of a mountain crest in a breathtaking setting.
To reach this peak, you have to take a strenuous fourty minute hiking trail at the end of Petra's center, following the old processional route, which involves 190-metre altitude difference on 1.6 km walk. But the climb is an amazing experience in itself, and the reward of the architectural/sculptural jewel on the summit (especially in the warm afternoon light) and the vastness of the mountains and valleys all around you are unforgettable.
Ad Deir, "The Monastery" in Arabic, – due to the crosses inscribed during its use as church in the Byzantine era – has a monumental dimension (it is 50 meters wide and nearly 40 meters high), with an 8-meter entrance portal. The upper order repeats the architectonic design of the Khazneh: a central tholos (circular structure) with a conic roof crowned by an urn, and two lateral structures topped with a broken (open) pediment). Not a mausoleum but a temple, probably in honor of Obodas I who reigned in the first century BC, and was deified after his death, Ad Deir is a striking example of a Nabataean reinterpretation of Hellenistic architecture.