The apartment is located just in the center of Madrid, in the exciting neighborhood of Lavapies. You can walk to a lot of the sights from here, although it was quite cold so we took the metro a few times.
Despite one previous review stating that there was an issue with the wifi we didn't have any problems at all with the connection.
Although the place is tiny it really has everything you need, even more than the usual stuff, such as body lotion in the bathroom and lots of teas in the kitchen.
The stairs aren't really a problem if you don't have an issue. But you should know what to expect.
The only problem is that the bed is extremely loud so every time you turn you feel like it's going to break. Aside from that, the apartment is probably one of the best places you can find in Madrid, and really good value for money.