I visited the Lobkowicz Palace with my girlfriend last year through the Prague card and we were astonished at the beauty of this site. The palace was built in the mid-16th century at the behest of the Lobkowicz noble family. Over the centuries the family accumulated an impressive collection of decorative arts objects, military and hunting equipment, musical artefacts, and classical paintings. The highlights of the collection are the Hay Harvester by the Flemish painter Pieter Brueghel the Elder and two beautiful views of London by the renowned Italian artist Canaletto. The collection includes numerous portraits of members of the European aristocracy from the Lobkowicz family, as well as their relations, but we didn't spend a lot of time looking at them as we didn't find them that interesting. The interior of the palace is absolutely spectacular and if you're a fan of classical music, it may be worth booking a combined museum and concert ticket for one of the classical concerts that take place in the Baroque concert hall in the early afternoon on a daily basis.
The palace is situated in the Prague Castle complex but it's very easy to miss, which would be a shame as it's definitely worth a visit. Make sure you listen to the excellent audio guide and take the opportunity to enjoy a beautiful view of Prague from the palace's terrace.