The 'leak' came from part of the wall that had begun crumbling from the side. From Garan's video, there were some undead that had been piling up. In time, they created some sort of flesh ramp, so several undead could go over the wall every day.
Since they were already there, Althea's group took down the leaks.
The Undead were not weak. They were as fast as a normal human at their level, or maybe even faster. Worse, they didn't feel pain. They also had better regeneration than humans.
Even if their bones get broken, they would recover soon. It usually took less than an hour, depending on the damage.
Interestingly, it was precisely because the Undead fell down from a height and broke a few bones that Althea's group could take advantage.
After they 'escaped' from the walls, the Undead would take several minutes to stand up again to continue on their way in random directions.
