The fortified city temporarily fell to the French invasion in 1672 (the Disaster Year); where the French invasion was only stopped west of Utrecht at the Old Hollandic Waterline. In 1674, only two years after the French left, the centre of Utrecht was struck by a tornado. The halt to building before construction of flying buttresses in the 15th century now proved to be the undoing of the central section of the cathedral of St Martin church which collapsed; creating the current Dom square between the tower and choir. In 1713, Utrecht hosted one of the first international peace negotiations when the Treaty of Utrecht settled the War of the Spanish Succession. Since 1723 Utrecht became the centre of the non-Roman Old Catholic Churches in the world.
Since 1723 what has Utrecht become  (If the question is unanswerable, say "unanswerable")
Utrecht became the centre of the non-Roman Old Catholic Churches in the world