Final answer:
The architectural achievement of Justinian was the Hagia Sophia, a grand church located in Istanbul, Turkey, originally built in 537 CE and featuring elements of Byzantine architecture.
Step-by-step explanation:
The architectural achievement of Justinian was The Hagia Sophia. It was a grand church located in Istanbul, Turkey, originally built in 537 CE and has served as a Greek Orthodox church, mosque, basilica, and now a museum. The cathedral incorporated elements of Byzantine architecture and featured a commanding size, with a central dome as the principal emphasis. Justinian wanted the church to be the biggest and grandest, so the builder appropriated materials from other ancient structures.