Testudo, the Latin name for a tortoise, was a military maneouver for defense or attacking ramparts that was used by the Romans. Visually, it resembles the plate pattern of a tortoise shell. The soldiers placed their shields overhead and next to each other with no space between. The wall (or tortoise shell) of shields prevented missiles from hitting the men. The testudo could be walked on or ridden across.

Here are some examples of its use:

"Though Marius had kept the attention of the Numidians, during the whole day, fixed on his attacks, yet, when he heard from his scouts how the Ligurian had succeeded, he animated his soldiers to fresh exertions, and he himself, advancing beyond the vineae, and causing a testudo to be formed,[280] came up close under the walls, annoying the enemy, at the same time, with his engines, archers, and slingers, from a distance."
Sallust Jugurthine War

"The Gauls' mode of besieging is the same as that of the Belgae: when after having drawn a large number of men around the whole of the fortifications, stones have begun to be cast against the wall on all sides, and the wall has been stript of its defenders, [then], forming a testudo, they advance to the gates and undermine the wall: which was easily effected on this occasion; for while so large a number were casting stones and darts, no one was able to maintain his position upon the wall."
Caesar's Gallic Wars Commentary Book II
"The idea uppermost in the minds of both parties is, that the present is the time in which they would have the fairest opportunity of making a struggle; the Gauls despairing of all safety, unless they should succeed in forcing the lines: the Romans expecting an end to all their labours if they should gain the day. The principal struggle is at the upper lines, to which, we have said, Vergasillaunus was sent. The least elevation of ground, added to a declivity, exercises a momentous influence. Some are casting missiles, others, forming a testudo, advance to the attack; fresh men by turns relieve the wearied. "
Book VII

"But the savages, thrown into disorder by their violence and anger, flamed up like fire, and hacked with repeated strokes of their swords at the close-jointed array of shields, which protected our men like a tortoise-formation."
Ammianus Macellinus XVI.12.44

References:

• A Dictionary of Military History and the Art of War, by André Corvisier, John Childs
• Soldiers and Ghosts: A History of Battle in Classical Antiquity, by J. E. Lendon
• A Source Book of Roman History by Dana Carleton Munro (1904)