The cathedral at the heart of the Mother See is the spiritual centre of the Armenian Apostolic Church and a UNESCO World Heritage site. The building you see today is layered — a fourth-century core, seventh-century rebuilding, and steady refinement of frescoes, belltower and dome over the following centuries. The interior is darker and more intimate than the white-stone exterior would suggest, with hanging silver lamps, a richly painted altar area, and faded medieval frescoes on the pillars.
The treasury museum behind the apse is the real draw for many visitors: it holds an extraordinary array of religious artefacts, including a fragment said to be from Noah's Ark, the Holy Lance reputedly used to pierce Christ's side, and reliquaries containing pieces of the True Cross. Whether or not you take the relics literally, the workmanship of the gold and enamel cases is remarkable. Photography in the treasury is restricted.
A major restoration covered the cathedral in scaffolding for much of the late 2010s; check ahead before counting on access to the interior.