Started well but ended very badly... upon arrival, the check-in process went smoothly and everything was more or less fine. There was a slight odour in the apartment but we didn't want to cause a fuss and so bought some incense which we burned to mask the smell. On the Sunday, we travelled to Sintra (which is beautiful and pretty easy to get to) and stayed the night but when we got back to the apartment on Monday at 5pm, there was an overwhelming smell of sewage in the apartment. It was awful. I spoke to the host, who was initially receptive and understanding and acknowledged that she, too, had noticed the strong smell that day when she'd entered the building. I asked the host if she could send a plumber around but she told me that this would not help as the problem stemmed from the building rather than the apartment. I told her that we would be going for dinner and would then make arrangements to leave the apartment if the smell had not gone. We got back to the apartment at around 10:30 / 11:00 pm and there was still a strong smell. I called the host again and explained the situation and told her we would be leaving the apartment. She was sorry and embarrassed but understood our position and seemed worried that I would leave a negative review. The conversation was friendly enough and I told her that we would expect a refund for the current night and the following night, which she agreed to. We then locked ourselves in the bedroom as the smell hadn't penetrated that far. And then, first thing in the morning, we packed our bags and left. The smell was still overwhelming - we couldn't enter the bathroom without gagging. We were making our way downstairs with our luggage to go check into a new place when the host stopped me as I was leaving the front door. I was expecting an apology in person but, instead, she proceeded to tell me that she would only refund 1 night (rather than 2) as AirBnB rules state that if a guest leaves the apartment or cancels the booking of their own accord then the host would not be liable and so the 1 night refund was a "good will gesture". This is preposterous and unacceptable - to expect us to stay in an apartment that reeked of sewage is beyond ridiculous. The host maintained that (and these are her words) "it is a direct result of a central sewage issue for which we have no control". Fair enough. That still doesn't mean that we should have to endure the awful smell. At this point I lost my temper as we were standing on the street with all of our luggage and I could not believe what I was being told. It is a shame as the location of the apartment is great and the big terrace is very nice in the sunshine. But the bathroom is far too small and the rest of the apartment is pretty underwhelming. It is clearly one of those Airbnbs that is run purely as a business for profit. And so I would urge anyone else who stays here and has a similar issue with the smell to mention it in their review rather than feeling like they shouldn't as that is the whole point of the review system and I can safely say that if I had known about this potential issue beforehand, I would've stayed elsewhere. I am not saying that the smell is this bad all of the time but it certainly is some of the time. Once we'd checked into the new place (which was only a couple of minutes away), the rest of our stay was amazing... Lisbon is a wonderful city and I cannot wait to go back.