Röslein auf der Heiden
I came across this question on a Russian forum, but there was no answer there. It is a line from the poem "Heidenröslein" by Goethe. I became interested in it too: why we have here "auf der Heiden", not "auf den Heiden" or " auf der Heide".

Question

Röslein auf der Heiden
I came across this question on a Russian forum, but there was no answer there. It is a line from the poem "Heidenröslein" by Goethe. I became interested in it too:  why we have here "auf der Heiden", not "auf den Heiden" or " auf der Heide".

Answer

It‘s Middle High German, dativ plural acutally "Heidenen",
It's a contraction of unstressed syllables with fading "e" after certain sounds or syllables.
Heidenen morphes into Heiden.
Mostly in Swabia and partially in Bavaria.

Answer

You might well ask! Probably it's archaic German.