I suppose I just haven’t read enough, but it has always seemed strange to me that after the Romans retreated to south of the Danube the Romanians kept a Romance language, while the lands the Romans kept ultimately adopted Slavic languages.
Until the Turks came in from the east, the Bulgars were the major longterm enemy of the Byzantines. The western Balkans were lost to various groups in succession, what is now Romania (Romanian has lots of loanwords from Slavic languages, as well as French and German) had fluctuating borders. I remember Kurt Gowdy talking about how in Romania, the universal nod for yes, shake for no was reversed as a consequence of having Turkish blades up against their throats with the question, do you accept Allah? :’)
http://www.omniglot.com/writing/romanian.htm
http://www.omniglot.com/writing/bulgarian.htm
https://en.wikipedia.org/wiki/Thrace
http://www.byzantium1200.com/aquaduct.html