The Testudo, or tortoise formation, was a technique used commonly by the Roman Legions during battles, particularly sieges. Testudo is the Latin word for "tortoise". The Greek term for this formation is "chelone" and during the Byzantine era, it seems to have evolved to what military manuals of the era call the "foulkon".
In the season finale, in the final battle, and after failing to maintain his Triple Axes (Triple line formation), to protect his army from the rebel archers, Marcus Crassus orders his front unit to use the formation known as Testudo. However, Spartacus and his army lift hidden ladders from under the ground to move over the formation and renders the latter useless.
This is one of the many roman infantry tactics used by Marcus Crassus, as oposed to Gaius Glaber, who relied on superior numbers instead of strategy. Crassus has an special signal to order the formation of Testudo, as with any other tactic.
To perform a successful and durable Testudo, the soldiers must use a Scutum shield, which helps maintain the integrity of the formation.
The Testudo can be disintegrated by using unconventional methods such as what Spartacus did in the series finale with the ladders, or by hitting the formation with high-density projectiles such as flaming boulders.
- Testudo at Wikipedia