How would you propose I do that? I have no idea how the engineer class works so its very hard for me to design areas for it, so far they've just kinda happened.
basically and especially in the last part of the stage 2, the issue is the following :
all "protected" sentry spots are useless.
all useful sentry spots are exposed
An engie can make its sentry "tank" in certain condition (himself not exposed, enough metal, regular but not large damage dealt). This is very effective against snipers, soldiers and heavies from a distance. It does not work against pyros and demomen.
But in the case of this stage, blu attackers have many angles to shoot from, starting with the sniper hideout in front of the end, the upstair exist on the left (red wise) and the main passage. There is one good spot against the main attackers, behind the rock (with the ammo and life layed near). But this spot is virtually useless once a demoman shows in the upster balcony and there isn't any other spot able to protect this one.
As an engineer, I had to try to set up the sentries behind the rock and the small wooden structure that swallows the cart. But from this angle, attackers can take cover behind the cart without any difficulty. To tell the truth we have a handful of uber demoment on our server and this doesn't help at all. They basically nullify the defense on this spot and the start of stage 3.