This question might have been answered by the thread about a Great Wall in Carolingian Europe, but this specific thought just came into my head. What if, anytime between the Roman withdrawal from Dacia and either Basil II's conquest of Bulgaria or establishment of the Second Bulgarian Empire under the Asen brothers, the Romans or Byzantines had built a wall across their Danube border from the Black Sea to either Moesia Superior/Serbia or Pannonia/Croatia & Hungary?
Something similar to the Anastasian Wall, but facing the Danube on the Roman side.
Out of curiosity, if properly manned and maintained, what effects could such a wall have against the Goths, Huns, Gepids, Avars, Bulgars, Magyars, Pechnegs, Cumans and Mongols, to name a few?
Any thoughts? And please, no Trump jokes.