Villa Cassandra is the perfect stay in Symi! (We come here every year, so it was a really nice experience to stay in an Airbnb.)
It was ten times better than expected and even larger than what you expect, when you see the photos. Really lovely interior, well equipped kitchen and there was a cleaning lady, who brought us new towels more than once (while our two weeks stay).
Would recommend to take the bus (it‘s 1,70€), which stops at the windmills, only a few meters away from the Villa. If you want to go downstairs you can also take the Kalistrata (a lot of steps, but not steep), which takes you about 15 minutes.
Perferably you could also rent a motorbike, we payed 15€ per day.
The highlight were definitely the big terasses! One leads off from the kitchen, very nice if you want to eat outside and the other one is on the roof. You also have the ability to sleep outside, because there are two big benches/beds at the rooftop terasse.
(Really only downside: People with back problems could have issues with the mattresses)
We would really recommend Villa Cassandra!!