Martin's Apartment is a real gem. It's tiny but you have everything you need and you're right in the very heart of Lisbon. I stayed there by myself for a couple of nights and loved the fact that I could just walk everywhere. I think the apartment also works for a couple who doesn't mind being very close ;) Otherwise it might be a bit small but you can clearly see that in the pictures. The tiny but lovely balcony is the cherry on top! Not sure if two people fit there, though.
Communication with Martin was easy and quick. He even proactively texted me when the weather changed and it was about to rain and told me where the umbrella was. Amazing! :)
The only category in which I didn't give a 5/5 is pricing, not because I think the apartment is not amazing, but it is in fact small, and the 200 Euro per night might be "normal" for Lisbon during that time (I did compare and see other prices were even higher), but I still think it's a bit much for the size of the apartment, but then again I'd probably give no apartment in Lisbon the full 5/5 in that category, since they all seemed quite expensive. :)