[Agema]

Ancient Greek did not have a 'c', just kappa. Although kappa is often translated into English via the letter 'c', pronunciation should always be hard like a 'k'. In the case of Arcoscephale, all the vowels should be short (as in pAt, pOt, pEt). I think it should pronounce as 'Arkoskefaly'.

As for what it means...

"-cephale" would refer to head, as pointed out. "Arcos" would not mean arches or bridges as that's a Latin root. I think the nearest you could get to "Arcos" would probably be "arch-" rather than "arc-". The root is to be first or come before; an Archon was an ancient Greek ruler or magistrate for instance.

Hence Arcoscephale would mean literally 'head first'. You could interpret that several ways with artistic licence, but I like to do it as 'The mind is paramount'. I think it suits a nation who put great store in philosophy, science and other mental pursuits.