But every continents has mountains. Still, if you take France as an example of one of the most “protected” European countries (far in the West, surrounded by mountains and seas for most of its borders), you can still see so many migration waves:

  - first arrival of hominids evolving into Neandertal;
  - Cro-Magnon arriving from the Middle-East;
  - Bell-beaker culture, probably from the Pontic stepp;
  - Celts (maybe from the West, probably from the East);
  - Romans;
  - the crucible of Northern, Germanic and steppe populations (Franks, Saxons, Goths, Wisigoths, Alamans, ...) that were the Great Invasions;
  - Celts again in Brittany;
  - Northern-African incursions and the whole Mediterranean melting pot on the southern seashores;
  - All the 20th century immigration waves (Poles, White Russians, Italians, Spaniards, Algerians, Moroccans, ...);
  - and probably many others I forgot/don't know about.

And even in Antiquity/Middle-Age Italy, you have Gauls and Celts in the North (after all, Northern Italy was Cisalpine *Gaul*, not Northern Italia), many Italic populations (Sabines, Etruscans, Umbrian, ...), a lot of Greek people in the South, then the Romans; after them all the aforementioned Eastern populations, then the Vikings demesnes, then Germanic incursions and settlements, etc.