Complex sentence: He did minor works for wealthy patrons in and around Oslo, but his major achievement was the renovation of the only seat of higher education in Christiania, the Oslo Katedralskole, completed in 1800.
Split into simpler sentences: He did minor works for wealthy patrons in and around Oslo. His major achievement was the renovation of the only seat of higher education in Christiania, the Oslo Katedralskole, completed in 1800.