From 23C3 Public Wiki
As in the last years, we will be able to provide a cheap sleeping accommodation in a gym in the vicinity of the Congress.
In order to be allowed in, you have to buy a ticket at the cashier at the entrance of 23C3. You need to buy a ticket for each night. You can buy tickets for every night. This time, we will only be able to provide the gym until December 30th at 12:00h. So you have to make sure you leave before that time.
Admission price will be 5 EUR per night. There are no discounts for multiple days.
We provide no reservation at all and we sell tickets on a first-come-first-serve basis. However, as we have again the big gym we do not expect any shortage.
The gym is located at Neue Roßstrasse in Berlin-Mitte, close to the U-Bahn station "Märkisches Museum"