Compare it to other maps, there is nothing wrong with 1:20 to reach the flag. Compare it to half the native maps, takes the same time there.
Actually, attackers already spawn inside the outer walls. It happened to me twice yesterday.
... I may request that it be removed from rotation. I'm pretty sure its easily the worst map on rotation and its pretty hated, to the point that some people actually drop server whenever it comes up.
In either case I have another siege map i'm working on which is loosely based on a real castle and I am trying to not make the same mistakes as with snowfall.