We stayed at this place for 1 night during a long layover. It is simply beautiful, and the location is fantastically pretty (within the walls of the fort I think, and RIGHT next to the castle keep!). I have been to many AirBnBs and this was easily the most romantic and atmospheric of any I have been to. If you want to surprise your bf/gf/partner, book them an evening here. The little balcony looks down a small cobblestone street, and you can see the castle towers from it!
I spent a lot of time wondering if Ana, the owner, was a textile artist, because her choice of linens, blankets, etc. was just superb (weird thing to call out I know, but I noticed it immediately). The bed was wonderful too- not too firm or soft. Ana left us food for breakfast and a bottle of wine, which was much appreciated! She greeted us when we arrived, and was kind, efficient (important when it's late and you've just left a long flight), and welcoming.
Do note that it's perched on quite a steep hill, but if you're an avid walker, you'll probably not mind and besides, the neighborhood is very quiet and cute. We went to Chapito for a late dinner and it was absolutely lovely- the views throughout the neighborhood are just amazing.
My only words of caution would be to make sure you get into the taxi line at Lisbon airport ASAP. I tried to find the mentioned departures floor taxis and couldn't. Make sure your taxi driver puts the address in exactly on his phone- otherwise he may wing it and get a bit lost (ours did).
Also, make sure the apt building door at the bottom of the stairs is unlocked before you leave the key in the apartment! If it's locked and you don't have the key, you won't be able to get out (we had a friendly neighbor help us).
We had a wonderful time here, and I would love to stay again. Lisbon is a beautiful city, I was glad to catch a glimpse of her!