I understand the spikes could be used as weapond but what about the round bosses found on celtic and viking sheilds?

Behind bosses, the shield had an opening through the wood (generally). This would allow the hand to have a grip that was located on the surface of the shield, and the knuckles would go into the area of the hole, which the boss protected... So, it was hand protection.

The popular boss reconstruction on bronze age shields, though, is to have no hole. This is because it as assumed that the boss was merely there for added protection in case if a spear, etc... Were to penetrate the hide/wood in front of the hand.

-Gregory-

(EDITED: Spelling errors!)

Thank You!