I had just one day in Dresden as a stopover between flights. I decided to stay in the very heart of the city and I think I indeed managed to see a lot of the most important places. I chose this place for the spa as well, among other things, and it actually turned out it's open all day. (took me some time to find it, I followed the "beauty suite" directions first but that's for treatment, not regular jacuzzi/sauna) The only minor disappointment is that you pay a very small fee to use it, which wasn't mentioned anywhere.