In years of travel and dozens of Airbnbs I have only had two very bad experiences - this was one of them.
The flat smells like sewage as soon as you walk in the door, making it impossible for me to stay, and the host was difficult with the refunds, saying I was exaggerating even when all sources in a simple Google search point to the fact that sewage smell is not only disgusting but also toxic:
"If untreated, sewage gases can leak high amounts of methane into your home, leading to headaches, weakness, nausea, vomiting, dizziness, loss of coordination, loss of consciousness and even suffocation. Unusual sewer smells are not something to ignore..."
There is a scent machine on timer, which masks the smell for all of twenty seconds, and which points to the fact that she is aware this sewage smell (etc) is an issue... In fact, I had specifically asked the host if they had dealt with the this since another guest had mentioned it, and I was in Istanbul for work: she said it was being dealt with - but it was not.
I got there at 8pm after a long flight and had to deal with bad communication, a difficult host, and finding a new place - the last thing I wanted to deal with upon arrival to a new city . Fortunately at least Airbnb staff was supportive.
Besides that, the building - although in a centric surrounding area - is quite dingy and the view from the main window of the apartment is of another building full of debris and construction garbage.
There are fantastic places for great prices in Istanbul. Do yourself a favor and stay elsewhere.