Why did Constantine protect and fortify Constantinople?

Because he wanted to strengthen the defense of the city.

Explanation:

Emperor Constantine moved the capital of the Roman Empire to the East in 324 AD, and this event marks the beginning of Byzantine history. The emperor's motives for such an act were mostly military, for many of powerful  enemies - Persia (around the Euphrates),Germans (on the Danube) and German pirates in the Black Sea  posed a threat to the Eastern provinces, which were the most populous and richest part of the Roman Empire. Constantine chose the ancient Greek colony - Byzantium, which had a great strategic position on the Bosphorus, as the new capital. The official name of the new city was New Rome, but it was soon named Constantinople by its founder Constantinople had an excellent natural harbor, surrounded on three sides by the sea, which contributed to security. Due to the increasing pressure of the various invaders, the emperors after  Constantine will build a complex system of triple city walls, which will resist the invaders until the fateful 1453.