I found the apartment very comfortable and the location very inspiring.
If the pictures make you think this is a cozy, cute apartment in a fantastic location, then you know what you're in for. I found the layout of the apartment to be just perfect, making it seem larger than it is, but still retaining the cozy, comfortable feel. And there's something amazing about looking out one window and seeing the expanse of the Tejo River, then looking out the other onto castle walls not far from how they would have appeared 900 years ago.
Two things just to be aware of (as other travellers have pointed out):
1) there are 64 stairs up to the apartment, some of which are taller than standard height.
2) the location at the top of the building means the ceilings slope dramatically. I'm just under 6 feet (1m80) and it wasn't a problem for me.
I propped open the windows on both sides and took advantage of the breeze, and found it comfortable even as outside temperatures rose.
I was also pleasantly surprised (given my experience in other upper-floor apartments in Lisbon) at the water pressure.
The apartment is recently remodeled, and is in very good condition.
I found the location ideal. It's within the walls of the Castelo São Jorge, so you're in one of the highest apartments atop one of the highest hills in Lisbon. There is a bus that will take you down to the central plazas, but if you don't mind steps, it's probably faster to walk.
I loved the place, and if my visa hadn't run out, I'd be there still!