I think it should be reasonably defensible. It's not just a failure of the architecture though. The tactics used were mostly ineffective. Defensive forces were positioned thinly with few real blockades. I saw one instance of police blocking an entrance through sheer mass and it was effective. The cops could have been done the same in the interior hallways instead of giving up their spread out weak positions. This would have prevented the use of firearms.

If you can stomach it, I ask everyone to watch the full lead up to the shot. It is an absolute failure of policing. Start at 37:20 on https://banned.video/watch?id=5ff6857e00bac0328da8e888 and yes it's NSFL (shot at 39:10).

It's a target - plain and symbol. It was the target of the 4th plane on 9/11, and it will likely be a target again.

Given the importance of the building and the people inside, it absolutely needs to be protected.

Is it sad that we need to protect things that are important to us? Yes. Should we do it anyway? Yes.