A Roman Legionnaire's shield was known as a Scutum. The usual shape was a curved rectangle as shown here. It was made from wood with a cloth covering and a brass rim.
The emblem on this example is the Labarum, Christ's initials. Just before the battle of the Milvian Bridge in AD312 the Emperor Constantine had a vision in which he saw the symbol in the sky with the words "By this, conquer".
Constantine ordered that every man should place the symbol on his shield to ensure victory against Maxentius. Even though his army was victorious Constantine remained a pagan until AD337 when he was baptised on his deathbed.