Sleep Cheap was a great hostel for doing exactly that - finding a cheap place to sleep. It's a u-Bahn out of the city centre (maybe 15 minutes) but for a good price that's really fine. The rooms were good - comfy beds and super clean showers. It is however right next to a main road which had permanent sirens going down it - couldn't hear them in my room but friends were bothered by it. The best thing for us was the people we met there - all awesome and ended up carrying on our travels together!
Hostel olet is about 10 minutes out of the centre on the tram or maybe a 20-30 minute walk. This sounds like an inconvenience but it's fine particularly given the good price. It's super cool as the hostel is in an elementary school - quite bizarre having kids' artwork on the walls but cool. The only concern was that the doors don't have locks on them - security is great at reception and you can hire lockers but I think bedroom doors should be lockable. Bring a sleeping bag or pay for a blanket!
I love Treestyle! It is in the perfect location with loads of bars around and less than 5 min walk in to the city centre. The staff are awesome and so helpful with recommending things etc. We arrived in the middle of the night which was absolutely no problem for them. We met so many awesome people staying at Treestyle - it was so easy to get to know people because of the rooms, communal areas etc. It is absolutely amazing for the price!