Final answer:
The Hagia Sophia was one of Justinian's most important architectural achievements due to its architectural masterpiece, including a large domed roof and mosaics, and its use of imported materials.
Step-by-step explanation:
The Hagia Sophia, one of Justinian's most important architectural achievements, holds significance for several reasons. Firstly, it is an architectural masterpiece that remains standing today, showcasing the grandeur and skill of Byzantine architecture. Secondly, it features a large domed roof and several mosaics, which contribute to its aesthetic appeal and artistic value. Lastly, Justinian's use of imported materials, such as columns and bricks from various regions, demonstrates his ambition to make the church the biggest and grandest in the world.