Great apartment! The door lock has a password, so there's no problems with keys and stuff, makes everything easier (specially checking in and out). The house almost feels like a bunker, being underground and with a small door, the atmosphere is really awesome. The apartment had everything we needed, 6 people could comfortably fit there for days.
Be ready to take the tram, since the main attractions are not at a walking distance. It's not much of a problem though, since it doesn't take that long to get to places and the station is across the street. It's a 20 minute ride to the train station, which helped us a lot.
Erman is a great host, he answered text messages very fast and was really helpful. He sent us a video of the lock so we could do it ourselves and it saved us, since we got there at 2 am and wouldn't have figured it out. We had a great stay and definitely would book this place again!