How did invaders access Bulgaria?

The most frequent and typical form of invasion was the river crossing. All three big rivers — the Danube, the Morava and the Struma, were widely used as such natural corridors. Their valleys led in the very heart of the Balkan peninsula, as well as in the centre of the Bulgarian state. For centuries the Danube played a role as a key frontier of Europe, the natural border of the Roman and Byzantine Empires and of Western Christendom, respectively, against incursions from the east and south-east, as well as a frontier of the Bulgarian state against the West and the invasions from the north-west.