Today I was messing around, delaying to weaken Athens. Well, I landed on Crete and took one city on the east end and met the red "no crossing" border.

So I make an alliance with Crete. Turns out their cities are on the opposite side of the red "no crossing" box. Wonder if I can use that to conquer the rest of the map.

which campaign was this? Spartan or philip?

Have you ever seen a red border after Bardylis' death in the Philip camapign?

Have you ever seen a red border after Bardylis' death in the Philip camapign?
I don't think so, the game unlocks everything for you.

It was the sparta campaign, yes.