We stayed at Hidden Treasure in the Castle for New Years 2019 ad had a fantastic time. It's very well located: you can be at be buzzing Chiado area in only a few minutes walk and there's lots of amenities and great restaurants in the immediate area) but it's also very quiet at night, with great view of the city. The Pingo Doce supermarket just down the road (and down the elevator) was super convenient.
The apartment itself was much larger than we expected with a large bedroom and living area, a fully equipped kitchen and a lovely little terrace to appreciate the views. Having 2 bathrooms was also very handy.
Property was very clean when we checked-in and the check-in process itself was super easy and well organised. Communication with the hosts was great and they were very quick to respond.
The building seems to have been recently renovated also, with 2 elevators servicing this 5 storey building.
The only thing worth mentioning is that since the property is located within the old town (where the Castle is) it's quite difficult to get taxis or ubers to drive right to the door as the streets seem to be closed off to non-resident's vehicles and we had to walk up a rather steep hill from where the taxis often dropped up off to reach the building. We didn't mind but for those with mobility issues this may be a challenge.
Overall we had a great time and highly recommend this place!