Basillica is only 40 m NW from Hagia entrance just walk a cross a street and you will see. Lines are not that long, like Hagia you can only pay with 20TL. No CC or other currency. Security check is just quick opening of bags and walking down the stairs to see the main cistern. It is very dark down with lots of spot lights but still very dark. Medusa head columns are towards the back part. Cistern is mostly empty except at the back park and had some fishes in them.