In the 20th century many states restricted the capacity of their populations to emigrate. Most famously, Churchill described the variety of emigration, financial and political restrictions in Central Europe—imposed by the Soviet Union—as an "iron curtain" due to their cohesive and impervious nature. As a result, pervasive emigration restrictions are interesting—what's their history?
Have there been other examples in history (say, up to the 19th century) where countries did not allow their citizens to emigrate or travel abroad, save by very special permit?
The only example that comes to my mind is Frederick II prohibiting study in foreign universities, but I am thinking more of blanket prohibitions.
EDIT: The answers so far are very interesting but I am mainly interested here in external passports.