On the legionnaire's scutum, there is a round iron boss in the centre. It is used for defence as it is big enough to shield blows, and can also stun or knock people over.
The design on the front of the shield is of eagle's wings and thunderbolts, sacred symbols of Jupiter.
Testudo is when ranks of soldiers with legionnaire's scutum lock shield to form an impenetrable shield wall, as protection from arrows. It is a traditional tactic used by the Roman Army.