The Roman helmet is pretty basic as helmets go. It protects the cranium, has flared out piece that stretches over the neck and to cheek pieces on hinges to protect the sides of the face without covering the ears. It does not cover the face at all. The issue as with most ancient armies is being able to hear and see commands and directions on the battlefield. The only real change for the helmet is when you get into the non-commissioned officer and officer ranks which had plumes on the helmets to distinguish rank. It allowed a soldier or commander to see where his officers were simply by looking for the plume on the helmet. The basic idea of the helmet is that it offered basic protection for the head but did not interfere with a soldiers ability to see or hear what was going on around them.
The fact that Paul would take this piece of hardware and equate it to salvation seems to be a natural fit. In many respects salvation keeps the senses free while still protecting the person. It truly is the way we should consider the gift of salvation. On the one hand salvation changes our mind and allows us to see and hear. Until salvation came to our lives, we were metaphorically blind and deaf to God, but once salvation comes we can see and hear. On the other hand, salvation does allow us to be mindful and protective of our thoughts. Our thoughts are reshaped as the blood of Christ cleanses our conscience from dead works to serve the living God. Salvation is the protector of our thought life and also allows us to hear God and see his direction.
In battle the Roman soldier needed this dual function. He needed to see his enemy, his friends and the directions of his officers. He needed to hear the voice of commands and hear when his friends might need his help. He also needed some protection for his brain because a head blow would end their involvement in the battle. In the same way Salvation does the same for us. It allows us to see our enemy clearly, hear the voice of our commander and be protected against the falsity of false salvation.
The helmet of salvation also completes the protection of the Christian soldier. The defensive portion of the armor is done in large part. We have truth, righteousness, the gospel of peace, faith, and salvation forming protecting us and yet also allowing the soldier of Christ to move in the joy and freedom of Christ for a purpose. The problem is, other than the shield which was used in a limited offensive capacity, most of this is designed for freedom of movement and protection. The sole purpose of all this freedom and protection was to put the soldier in a position and survive long enough to do the enemy some damage. Protected and free the soldier now can offensive.