Thin iron/steel sheets.

[Dan Howard]

So why would they start to present soldiers in fantastic armor when befire they were shown in real one? One could say that it was hellenistic influence but if so, why armor are plain and simple? Why not make medusa theme on them? Why not make clearly visible muscles like Greek once? There are 2 answers for that.

You do realise that Roman sculpture and stonework was painted? A more obvious answer to your question about plain armour is that all of the decorative detail was in the paint, which has now been lost.