I strongly recommend Asli's appartment.
It is perfectly located, quiet though very close to all the must sees. There are many very nice restaurants withing walking distance which allows you to relax knowing that you won't have a long trip back home. Don't miss the Hammam in the same block !
The appartment in itself is very cosy, comfortable and spacious and you can tell Asli knows something about fashion and furnishing !
Asli is also a wonderful host as she waited for me and my friend to check in when we were slowed down at the airport. She is very reactive when you have questions too :)
Thanks to all that we had 4 amazing days in Istanbul.