There have been a surprising (given the terrain) number of (known) invasions of the Balkans; the Romans under Trajan crossed the Danube and conquered Dacia, and grabbed a huge amount of loot in the process (gold mines I believe), the biggest jackpot they ever hit, and that was the financial peak of Imperial Rome. Other than that, it remained pretty quiet for a long while. Diocletian ended a long period of near-anarchy during the 3rd century, then set up a system of succession, built a self-sustaining miniature city / fortress to retire into, and retired to test his system. He also forced his eastern empire colleague to retire to make sure the system worked in the eastern division of the empire (Diocletian was the one who divided it formally). Anyway,
Diocletian's retirement mansion can still be visited today, in Split.