Many thanks indeed Shield and Spear. The quotation from Cassius Dio (describing the disaster at Carrhae in 53 B.C.E.) is very interesting. I am however slightly perplexed, as I found it in the fortieth book of Dio, the twentieth chapter (in Vol. III of Loeb's edition):
''When this [the death of P. Licinius Crassus, cut down with a few cavalrymen after being drawn out by a feigned retreat by a small number of Parthians] had taken place, the Roman infantry did not turn back, but valiantly joined battle with the Parthians to avenge his death. Yet they accomplished nothing worthy of themselves because of the enemy's numbers and tactics, and particularly because Abgarus was plotting against them. For if they decided to lock shields for the purpose of avoiding the arrows by the closeness of their array, the pikemen were upon them with a rush, striking down some, and at least scattering the others; and if they extended their ranks to avoid this, they would be struck with the arrows. Hereupon many died from fright at the very charge of the pikemen, and many perished hemmed in by the horsemen. Others were knocked over by the pikes or were carried off transfixed. The missiles falling thick upon them from all sides at once struck down many by a mortal blow, rendered many useless for battle, and caused distress to all. They flew into their eyes and pierced their hands and all the other parts of their body and, penetrating their armour, deprived them of their protection and compelled them to expose themselves to each new missile. 5 Thus, while a man was guarding against arrows or pulling out one that had stuck fast he received more wounds, one after another. Consequently it was impracticable for them to move, and impracticable to remain at rest. Neither course afforded them safety but each was fraught with destruction, the one because it was out of their power, and the other because they were then more easily wounded.''
The TESTVDO itself and its uses are also described by Cassius Dio, when treating of Mark Anthony's Parthian campaign in 36 B.C.E, in the twenty-eighth chapter of the forty-ninth book:
''One day, when they fell into an ambush and were being struck by dense showers of arrows, they suddenly formed the testudo by joining their shields, and rested their left knees on the ground. The barbarians, who had never seen anything of the kind before, thought that they had fallen from their wounds and needed only one finishing blow; so they threw aside their bows, leaped from their horses, and drawing their daggers, came up close to put an end to them. At this the Romans sprang to their feet, extended their battle-line at the word of command, and confronting the foe face to face, fell upon them, each one upon the man nearest him, and cut down great numbers, since they were contending in full armour against unprotected men, men prepared against men off their guard, heavy infantry against archers, Romans against barbarians. All the survivors immediately retired and no one followed them thereafter.
This testudo and the way in which it is formed are as follows. The baggage animals, the light-armed troops, and the cavalry are placed in the centre of the army. The heavy-armed troops who use the oblong, curved, and cylindrical shields are drawn up around the outside, making a rectangular figure; and, facing outward and holding their arms at the ready, they enclose the rest. The others, who have flat shields, form a compact body in the centre and raise their shields over the heads of all the others, so that nothing but shields can be seen in every part of the phalanx alike and all the men by the density of the formation are under shelter from missiles. Indeed, it is so marvellously strong that men can walk upon it, and whenever they come to a narrow ravine, even horses and vehicles can be driven over it. Such is the plan of this formation, and for this reason it has received the name testudo, with reference both to its strength and to the excellent shelter it affords. They use it in two ways: either they approach some fort to assault it, often even enabling men to scale the very walls, or sometimes, when they are surrounded by archers, they all crouch together — even the horses being taught to kneel or lie down — and thereby cause the foe to think that they are exhausted; then, when the enemy draws near, they suddenly rise and throw them into consternation.''
My speciality is not the military, nor periods later than the Julio-Claudians so regrettably I cannot help you with the main question (the book seems a very good reference), but the full quotations might be useful. They do seem to indicate that the TESTVDO was used in battle as well as in siege -- Mr Nathan Ross and Mr. Dan Howard are far better qualified than I am, but Dio alludes to the TESTVDO being used ''under ambush'' and again ''when they are surrounded by archers'' as well as when ''they approach some fort to assault it''.''
Patrick J. Gray
'' Now. Close your eyes. It's but a short step to the boat, a short pull across the river.''
''And then?''
''And then, I promise you, you'll dream a different story altogether''
From ''I, Claudius'', by J. Pulman after R. Graves.