I feel kind of bad rating Vilnius only 3.5 stars because I really liked it but had to accept that it was very similar to countless other central European cities but without a real wow factor. If you like an eclectic mix of architecture Vilnius has Gothic, Renaissance and Baroque all cheek by jowl as well the cities ancient fortifications yet while it remains "interesting" it never hits the wow! in the way that, say, Riga does, or Krakow.
The city highlights are the Cathedral and its quirky, stand alone bell tower (pictured), the Gothic splendour of St Anne's Church and the countless baroque churches that seem to be present on every street.
Gediminos tower, one of the cities old fortifications, affords an excellent rooftop view of the city while the tower itself is no great shakes. 
Maybe I'm being overly harsh, like I said, I DID enjoy being there, it's a pleasant break but just don't expect too much.