The Danube is one of the longest rivers in Europe, stretching 1,785 miles from the Black Forest (Germany) to the Black Sea. It forms the majority of the border between Romania and Bulgaria, which had languished under Ottoman rule for five hundred years. Hence, for centuries the Danube had indeed been the "frontier of Turkey-land", although Ottoman control and influence frequently extended north of it.
By the time of Dracula, things were starting to change south of the river. The Russo-Turkish War of 1877-78 had led to the liberation of part of Bulgaria. The Principality of Bulgaria, though nominally a vassal state of the Ottoman Empire, was now effectively independent. The restored and enlarged nation finally declared full independence in 1908, pushing the decrepit empire back close to the border of modern Turkey.