Bologna has retained a marked medieval aspect; many streets are arcaded. Noteworthy structures include the Palazzo Comunale (13th and 15th–16th cent.); the Renaissance-style Palazzo del Podesta; the palace of King Enzio (13th cent.); the Basilica of San Petronio (begun in 1390), with a 15th-century doorway by Jacopo della Quercia; the Church of Santo Stefano; the Church of San Giacomo Maggiore (founded 1267, major alterations in the 15th cent.); the Church of San Domenico (early 13th cent.); and the Archiginnasio (once the seat of the city's noted university and now a library). Bologna also has an archaeological museum; an art gallery, with works by Bolognese artists, including Francia, the Carracci, and Guido Reni; and a nuclear research institute. The city's observatory (founded 1712) is the oldest in Italy. On hills near the city are the Renaissance Church of San Michele (in Bosco) and a former Carthusian monastery.