A marvelous mashup of historic and modern architectural treasures awaits visitors to Szczecin, a busy Baltic Sea port tucked into the northwest corner of Poland.   The city’s medieval castle reminded me of a wedding cake frosted with cream cheese. Its futuristic concert hall looks like an ice palace with jagged glass panels that light up the sky.   Szczecin (pronounced shteh-cheen) may not be a well-known tourist destination—yet. However, the city of 400,000 impressed me with its eclectic attractions, close proximity to Baltic beaches and innovative take on traditional...