That is not Akkusativ, but Dativ. This form was more common in the past and survives in some constructions, such as:
Auf Seiten, Auf Erden, in Sachen, von der Weiden (last name)
as well as in regional expressions:
Auf der Wiesen
Examples help us grasp that no plural is involved. Consider these:
Die Hölle auf Erden. / Hell on Earth.
Nobody is interested in imaginary new Earths here! Similarly, take this example from Wiktionary:
Die Mitbischöfe Anselms standen in dieser Auseinandersetzung mehrheitlich auf Seiten des Königs. / The majority of Anselms fellow bishops sided with the king in this disagreement.
Again, the king's side is one and is contrasted with his adversary's side. Nobody talks of multiple sides (left and right) of the king here!
Other cases, where I am not absolutely sure a singular is involved, are:
zu Schanden kommen, zu Ehren kommen, , zu Ehren (z.B. des Königs)