To me, it was crowded with tourists. I heard a lot of the Queen's English (British tourists). But, I can't compare it to summer time. I am sure when you go, it won't be as crowded. Weather wise, it was cold and rainy while we were there. After we left to Krakow, it snowed, in Prague and Krakow. I arrived back to PHL, and it was 52 outside, and I felt warm! The highs were only in the 40s when we first arrived, and the highs were in the low 30s when we left. Brrrrr!!!!
The Marriott is a nice hotel. I highly recommend it, if you like the cleanliness of a Western Hotel. I would try the Hotel U Prince next time. It is right across from the astronomical clock in Old Town Square. I wish I spent more time in Prague. Take the Jewish Museum tour, very informative. Prague castle is nice too!