Fire Emblem Awakening [SPOILERS INSIDE]

There should be a marker though, like a bunch of tightly, bunched forts or usually the big area farthest from your starting position.

in some areas you can predict the reinforcements, in others (like the one that pissed me off) they came from the same place where you spawned, so if you hadn't charged away from it, you'd get ganged up by all the reinforcements.

an example is the map where you get cordelia, she suddenly appears (from the direction you came), says "the fort has fallen, enemy troops are coming this way"

and a turn or two later, WYVERNS, WYVERNS EVERYWHERE.