The location is good and you could go anywhere either by foot, metro or bus.
There are a lot of surprising people around and near the Martim Moniz square but there is no risk and globally a very safe place.
Flat is situated in a nice quite place in a historical part; you need to be fit walking around this area as it is very hilly.
The host was welcoming. I would recommend this place if you are on a tight budget and want to be in the heart of Lisbon.