Gripsholm Castle is beautifully situated by the water on the outskirts of Mariefred. A royal residence since the time of Gustav Vasa, it is befittingly regal and enchanting in appearance with its cupola-topped towers and opulent interior design. It boasts of a truly eventful history that spans close to 500 years by virtue of its association with the royal household. It was built by Gustav Vasa in the 1530s at the site of a 14th-century medieval fortress. Johan III was imprisoned here by his half-brother Erik XIV, and was later held captive here himself. Sweden's so-called "Theatre King," Gustav III, constructed a palace theater at Gripsholm, with a unique third balcony from where the servants could view the performances without being seen by the nobler members of the audience. Gustav III also laid the foundations for what is today known the State Portrait Collection; housed at the castle, the collection is composed of over 4000 portraits dating back to the 16th Century with a few that are even older. Renovations in the 17th and 18th Centuries only served to add more character to the royal palace that is now open to the public, although it is still technically owned by the crown.