A jewel of late-Victorian theatre architecture on Guildhall Walk, designed in 1884 by C J Phipps and remodelled by the great theatre architect Frank Matcham in 1900. The 500-seat auditorium retains all of its Matcham fittings — gilded plasterwork, cantilevered balconies, painted ceiling — and is one of the most complete Matcham interiors in the country.
It was nearly demolished in the 1970s, saved by a long campaign, and now hosts touring productions, opera, and Portsmouth University drama performances. Even if you don't catch a show, it sometimes runs daytime tours; check the website.