Conclusion: they face each other across Sultanahmet Square, five minutes apart — do both in one morning; pick Hagia Sophia for Byzantine history or the Blue Mosque for Ottoman tiles and free entry. - Hagia Sophia: completed AD 537, a Byzantine cathedral turned mosque; see the Deësis mosaic and the vast dome; foreign visitors usually take the paid upper-gallery route; allow 45–75 minutes. - Blue Mosque (Sultan Ahmed Mosque): 1609–1616, six minarets and blue İznik tiles; generally free, but it closes to visitors around prayer times (Friday midday is worst); allow 30–60 minutes. - Both require covered shoulders and knees, a headscarf for women, and shoes removed. - Route: visit whichever is open first, cross the square, then continue to the Basilica Cistern or Topkapı Palace.