Helder's place is really nicely located, close to Intendente Square/Martim Moniz metro stops, but I personally ended up exploring Lisbon mostly on foot, because it was such a good base to go almost anywhere: Alfama, Bairro Alto, Baixa, very close to Rossio (if you want to visit Sintra). It's located in an older building, but it provides all amenities, and Helder himself is very responsive in case of any issues or needs; he was also very accommodating with check-in/check-out times. A good experience!