BAYREUTH, Germany — This small city is known throughout the world for its summertime Wagner festival, founded in 1876 by Richard Wagner himself. But long before the “Ring” cycle, Bayreuth had another operatic visionary.

Wilhelmine, Margravine of Brandenburg-Bayreuth, was the eldest daughter of King Frederick William I of Prussia, and the sister of Frederick the Great. An ambitious polymath who composed music, wrote verse and corresponded with Voltaire, she built Bayreuth’s intimate yet elaborate Margravial Opera House, one of the most outstanding surviving examples of Baroque theater architecture in Europe.

On Tuesday, the nearly 300-year-old opera house — a Unesco World Heritage site since 2012 — reopens to the public after a six-year renovation that cost 29.6 million euros ($36.6 million) and returned its dazzling ornamental details, murals and trompe l’oeil effects to something approximating their original brilliance.