I had a wonderful stay in the City Circus Hostel ! The place is really well located, you are able to enjoy all of the citie's treasures by feet.
The rooms and common parts are clean, the lockers in the dorms are big, and it's possible to get a really good vegan/ vegetarian breakfast !
Special mention to Tryphon (or Triphon), who welcomed me, and helped me a lot during my stay (it was my first destination in Greece) ! Thank you, and go on with those professional and human qualities :D