We enjoyed our stay - it was a great value for a our one night stay in Madrid. This is a great location and very walkable to many places - I’d be cautious though if you are attempting to get a good nights sleep. The unit is indeed an “entire separate unit” but this apartment has been converted into separate units. The walls are quite thin and you share an entry/exit with other guests so it can be quite loud - particularly if you are assigned the unit near the main door. Bring ear plugs.
Otherwise it was a great stay and the unit was actually much nicer than the images - I guess it depends which unit you are assigned.
Easy check-in and well priced.