I am convinced that you will not find a better view in all of Lisbon. You can see the water from the living area and the castle from the bedrooms and bathrooms. It is amazing.
The location of the apartment is convenient. There are lots of good restaurants in close walking distance.
The flat is large and comfortable. As mentioned in the listing, this is Alex's home, so you will find his personal items in the flat. (I mention this only because some people don't like the idea of staying in someone else's home.)
Alex was a terrific host. Don't skip the in person meeting with him because he'll tell you how to work the appliances, etc. (Maybe not important if you are European, but nice for this American.)
Alex was kind enough to have wine, fruit, milk and butter in the fridge for us, plus coffee and sugar.
One important positive that you may not consider until you are in Lisbon is that the apartment is in a location where a taxi or Uber can drop you off and pick you up right at your door. This is not possible in some other locations of the city.