Although I've seen quite a lot of so-called "medieval cities" in Europe, Tallinn came as a surprise. Its whole Old City is without modern buildings or distractions. I had a great time just roaming around without a map, peeking around corners.

Highlight - though not medieval, dating from 1900 - for me was the Alexander Nevsky Cathedral. It's a wonderful Russian-Orthodox church that looks like a big birthdaycake