Karaköy
Galata is just north, across the Golden Horn, from the Old City of Istanbul. Further north is Beyoğlu: İstiklal Avenue is its thoroughfare, and adjoining Taksim Square is considered the heart of the city — if it were sensible to speak of a "downtown" for multicentric Istanbul, that would most likely refer to this area.
About Karaköy
Galata is just north, across the Golden Horn, from the Old City of Istanbul. Further north is Beyoğlu: İstiklal Avenue is its thoroughfare, and adjoining Taksim Square is considered the heart of the city — if it were sensible to speak of a "downtown" for multicentric Istanbul, that would most likely refer to this area. Besides the Sultanahmet/Old City area, this is where you will spend most time during your Istanbul trip — the district has its fair share of sights, dining, nightlife, and accommodation.