Regarding The Battle of Olympus, there is a path from Argolis to Phrygia, but it's very difficult since there is a huge pit and a platform. It is not possible to leave Argolis for Phrygia, or vice-versa. You can leave Phrygia for Argolis but upon entering Argolis, there's that platform which is impassable.

I am wondering if the developers of this game, Infinity, had run out of time before they were able to finish programming the passage between Argolis and Phrygia (meaning, was it ever meant to be passable)? This section can be accessed in Argolis using the Eye of Graeae while in the left cave (the one you enter to defeat the Lion of Nemea who had held Prometheus captive), and then going east in the same secret area you had accessed to save the Nymph that gave you the first of three fragments of Helene.

~Ben