Located across the Golden Horn is the Galata Tower (first photo, not so aesthetically pleasing but good for context). You can't really tell, but it's possible to go up to that top level (with the really rickety looking iron railing, yeah...), and so we did.
The first view is looking across at old (the oldest) Constantinople - on the very right you can see the edge of the Hagia Sophia! The final photo is looking out across the Bosphorus towards Asia. That's right - Asia. Worth going on the really scary walkway? Probably :)