Constantinople was the capital of Byzantine Empire and also was holy city for Christianity. Byzantine Emperors were aware of these facts so they have reconstructed this city several times.
Especially the cisterns had a great importance due to fulfill the water necessity of the capital. During a siege, cisterns had a vital role. Thus many cisterns were built. Three cisterns were a head. They were the biggest ones.
You should do it before getting old….. they are all worthvisiting…