This is one of the best apartments you will find in Lisbon. If it is available, grab it.
The terrace and its view are spectacular. When my wife, sister and brother in law were visiting the Castelo my daughters stayed behind to hang out on the terrace and we sent them pictures we took of them on the terrace from the Castelo. We were in Lisbon during a heat wave but in the late afternoon the terrace was cool and in the evenings - even a little cold with the breeze.
We had seven people in the apartment and my sister and brother were around throughout the day (slept in a smaller AirBnB nearby) for a total of nine most of the time and it still felt spacious. At any given time, particularly in the evenings, one or all of us were on the terrace. The air conditioning is 'US equivalent' - even when it was searingly hot outside the apartment was cool.
The apartment is very centrally located. We walked almost everywhere. There are lots of restaurants and shops within easy, easy walking distance. We would make it a point to walk to a local patisserie each afternoon to pick up breakfast for the morning. There is activity in Martim Moniz almost 24/7 and there are always cabs or Ubers available within minutes.
The host and support staff were great and worked with us to get us into the apartment even after our travel plans where shredded by the airlines (Summer 2022 airline debacles).