Conclusion: for a short sightseeing trip stay in Sultanahmet (near the monuments); for four days or more, base yourself in Beyoğlu or the Asian-side Kadıköy. ① Sultanahmet: walking distance to Hagia Sophia and the Blue Mosque, but it empties by 9pm and carries a 30–40% location premium. ② Beyoğlu/Galata: nightlife plus culture, with a 20-minute ferry to Kadıköy. ③ Kadıköy (Asian side): the best value — 20–30% cheaper than the European side with better restaurants, and the Bosphorus ferry crossing is part of the experience. ④ Budget: guesthouses $30–50/night, mid-range $70–120, luxury $300–800. ⑤ Best seasons are April–June and September–October — book 6–8 weeks ahead. ⑥ Airports: Istanbul Airport is about 60 min by metro; Sabiha Gökçen has no metro — the Havaş bus to Kadıköy takes ~45 min (200–250 TL).