The apartment was as expected from the pictures and the description. It is a quite small and basic apartment with a great location in a lesser known, multicultural part of Madrid. There's a huge supermarket open 24/7 right next to it, which is super convenient, and the kitchen has all the equipment you should need for a short stay.
Note that the apartment is really not well sound isolated and the street next door is quite busy, which could be a problem for light sleepers.
Marta was a very reactive host in general and made sure we had everything we needed during our stay. However the check in and the check out were quite confusing and not as smooth as hoped for despite the relevant communication beforehand. After waiting for 15-20 minutes both times, we were left to ourselves ring the host to know that the key was to be picked-up and left in a nearby garage, instead of given in-person at the apartment, as had been agreed on before.
Still though, an interesting and a little cheaper option for accommodation in Madrid, to discover the city and the Lavapiés neighborhood!