First stop - tram and metro to the Chora Museum. Here's an image of what it used to look like - it still does, but much of the building is currently under restoration so you can only see a small bit of the interior.
Still, I gasped out loud when I walked in.
The Chora is an ancient Greek Orthodox church that became a mosque and now is a museum rich with Byzantine mosaics, and also frescos, thought to be by the same artists. A sign of the times that I was almost alone here. One other person...
Things were so quiet that I snapped a photo of the police or security people at the entry. They scan bags, in a desultory way, as people come in. He indicated to me, very politely, that I was not allowed to photograph them. As he was saying this, there was some shouting outside. Action stations in an instant. He reached for his gun and whirled around and raced out, and so did his companion, followed by the shop attendant and several others as the shouting escalated.
Back to the Chora. It is in a suburban area just inside the second city walls, the ones built by Emperor Theodosios. Mostly apartment buildings in steep narrow streets that got narrower and poorer as I wandered down towards the Golden Horn (the real one, not the hotel of same name) and then back up again in search of another museum, the Fethiye.
Turns out this is the Ottoman Baroque Ahrida Synagogue and I'm in the heart of the historically Jewish area of Istanbul. Most of the women here are wearing full burka with faces covered and only eyes showing, and a lot of the men were wearing some sort of headgear.
I did eventually stumble across the Fethiye Museum. Here too you can only see a small part of the interior. More mosaics.