From arrival, there was a noticeable damp and musty smell that did not go away even after constant ventilation. The extractor fan in the bathroom was broken, which no doubt contributed to this (note that previous guests have raised this issue before). Despite trying to use bleach to clean the bathroom, and plugging in a separate fan, the smell did not disappear for the duration of our stay (10 days). The hosts were very bad at communicating about this issue and did not offer any solutions despite repeated requests. By the end of our stay, we both had aching throats and my partner developed a bad cough.
The flat is also on a ground floor with very little natural light, which means it is not an ideal place to stay if you are planning to be in the flat for any length of time.
We requested a partial refund from the hosts in light of the damp situation, but they were unresponsive. At this price range, you will have no trouble finding much better places to stay in Madrid.