by Anduin
what Vlad said!In addition: Keep in mind that the area with the stronghold is NOT considered empty (in contrast to what you wrote in the headline!). It is still an enemy area and your units cannot pass enemy areas when they move.
See it like this: The stronghold is definitely not empty just because there are no "units" of your opponent there. The stronghold itself has garrisons and can perfectly defend itself. Thats why it has a strength value! Your "units" are just there to strengthen the defense.