As other reviewers indicated, the stairs are no joke. I had a heavy bag because I was traveling for a few weeks in Europe and through different climates. I suffered carrying it through the narrow spiral stairs to get to the front door of the apartment, and then the internal set of stairs inside the apartment. The location is also a bit sketchy, so I recommend that you do some research to learn about Brussels' historical center before booking the place.
Transportation to the apartment is difficult since the street is for pedestrians only. I took a taxi from the bus station, and the driver could not find his way to the apartment. He was very frustrated, cursing in French, and hitting the dashboard, so I asked him to drop me off at a train station. He did not want to let me out of the car or get my bag until I paid him, which was pretty scary. I finally took a train ride (only one station away) and walked to the apartment. Save yourselves the scary taxi ride and just take the train to the apartment--it's a few feet away from the station. You can also ask to be dropped off at the Hotel Amigo, which is just around the corner and is more accessible (and I suggest taking an Uber instead of a cab).
Aside from the strange configuration and all the external and internal stairs, the apartment is functional and clean, and it is surprisingly quiet when you close the windows. The view of la Grand Place is nice. The beds are comfortable and clean. The check-in and out was very easy, and the hosts very responsive and helpful. However, don't expect any luxuries. The apartment is scarcely decorated, the sofas are a bit tired, and the floor is unfinished so you may get splinters if walking barefooted. I guess that's fine for the price because you are primarily paying for the location. I recommend this place if you want to be close to all the tourist attractions in Brussels and don't expect to spend much time at your rental.