The apartment was as expected in a central location in San Sebastian. Communication with Martin in advance was great with many great tipps from him. Onsite we met Tania who managed the appartment for Martin. She contacted us in advance that also the check-in worked smoothly and she also gave us great tipps and also was there so solve any issue we had.
The appartment in total is really great. Unfortunately, one of the beds in the bedroom was a bit broken (bad sound while moving at night during our sleep) so I had to use another bed in another room. We reported that to Tania and Martin and they promised to fix it as soon as possible.
Walking distance from the apartment to the city center is approx. 10 - 15 min. There's also a bus line next to the apartment and a bakery, a cake shop etc. So the location itself is more than good.
Also if you have a question Tania as well as Martin replied back immediately (we used WhatsApp) any time.
However: If you are sound sensitive when you want to sleep you have to know that in San Sebastian the garbage trucks come at night with lots of noise. If it's warm and you want to sleep with open windows this could wake you up several times (as they came also sometimes twice a night at 12am and 1am) - of course, apartment also has air conditioning but not in the small bedroom. But that's how they collect the garbage in San Sebastian so one needs to deal with that.
Thanks again to Martin, his wife and Tania!