Crests and Roman Soldiers

[M. CVRIVS ALEXANDER]

I might be wandering off topic a bit, but how long does it really take to put on a helmet? Does this mean they would normally put on some kind of arming cap or wrap rags around their heads first, or does it just mean that the helmets were in their tents or something?

It appears that the soldiers were engaged in fortifying the camp, so they would not have had their shields and helmets with them. As the enemy attacked with the great speed, they only managed to grab their shields as they were, with the covers still on, but not to do anything else.