Testudo formation

In Ancient Roman warfare, the testudo or tortoise formation was a formation used commonly by the Roman Legions during battles, particularly sieges. Testudo is the Latin word for "tortoise".

The testudo was used to protect soldiers from all types of missiles. It could be formed by immobile troops and troops on the march. The primary drawback to the formation was that, because of its density, the men found it more difficult to fight in hand-to-hand combat and because the men were required to move in unison, speed was sacrificed.

Excerpt from Wikipedia.