At this time, the powerhouses like Zaol and Gregor were focused on their area, unable to leave, though it was more like attack here and defend their situation.
They could lure enemies and pull them to the sentry areas, and they nabbed several enemies one by one like this. The enemies couldn't do the same, obviously, because the sentries were still active, not to mention the Alterrans specifically chose to camp in areas where at least 2 sentries were working.
Their priority was not to add another leak, so while they knew other areas were being attacked, they focused on their task because winning the war was simply to keep these people out.
In other words, it was a stalemate. The powerhouses were limited in their movements, yes, but so were the trebuchets and the remaining powerhouses that guarded them.
