The Unsullied are elite heavy infantry, trained and sold by the Ghiscari slaver-traders in Astapor.
Recruits are picked among slaves at the age of five, and they're chosen for their size, speed and strength. All Unsullied are eunuchs: They're cut young, and even their penises are removed in order to promote chastity. On the same day, each boy is given a puppy, and at the end of the first year he's required to strangle it (supposedly to assess their willpower and obedience). They also drink the so-called Wine of courage (1) from the day they're cut, to make them fearless and to raise their pain-threshold.
Unsullied aren't allowed to own anything, except their equipment and weapons, nor do they have a life outside their duty. They bear patronizing names (2) which are picked at random each day, to remind them of their low stature as individuals.
A company of Unsullied may consist of men of different origins and races, due to their slave-backgrounds. A majority of Unsullied are Dothraki and Lhazarene, but the Free Cities, Ghis, Qarth and the Summer Islands are represented as well (3).
Unsullied cost more than regular slave swordsmen, due to their extensive, long training. They're sold in units of 100 or 1.000 (4), and fully equipped, but the buyer has to set their own officers over them.
Recruits are trained every day, from dawn to dusk. The training is so rigorous that only one in three survives. It consists, among other things, of running (fully equipped), scaling of mountains at night and walking across a bed of coals. They're trained to fight in "lockstep formations" (5), in the fashion of the Old Ghiscari Empire. For protection against ambushes while on the march, they learn how to build field fortifications using stakes, protected by pits and ditches. These camp fortifications also serve as a refuge in case of retreat in battle.
In order to complete their training and win their spiked caps, each Unsullied must go to the slave marts and kill an infant before its mother's eyes (6). A fully trained Unsullied is typically 14-20 years old (7).
Although the Unsullied are highly trained and skilled, it's their strong discipline and obedience that really set them apart from other soldiers.
Armed with shortsword, shield and heavy spears.
Wears sandals, a quilted tunic, and a conical bronze helm topped with a sharpened spike a foot tall. Underneath, they wear white linen breechclouts.
The Unsullied are obviously fashioned after the soldiers of antiquity, most probably the Greek hoplites or Roman legionnaires (8). They're well equipped with spears, shortswords and shields, and are highly disciplined, drilled and trained, just like a Roman legionnaire. We're also told that they build field fortifications or entrenchments while on the march, an attribute which the Roman army also had a passion for.
They fight in either packed phalanx- or in superior, loose cohort-formations ("lockstep formations of the Old Empire"). Roman cohort-formations provided more space for maneuver and were consequently more flexible and responsive. This battle formation allowed the soldiers to take better advantage of their primary weapon, the shortsword (the Greeks relied on spears), and made them less vulnerable to flank attacks. During battle, they used to throw pilums (javelins) before closing in with shortswords and shield.
The Roman army is also well-known for its administrative organization, but the organization of the Unsullied seems far less sophisticated in comparison. The buyer is supposed to integrate the troops into their own military organization and doctrine, and has to set his own officers over them. But in each unit (Unsullied are sold by the hundreds or thousands) there are probably several lower-ranked officers or instructors who are sold together with their unit, in order to preserve the integrity of the group.
(1) "It's no true wine at all, but made from deadly nightshade, bloodfly larva, black lotus root, and many secret things.", Kraznys mo Nakloz (3p261).
(2) Some may find their names more amusing than patronizing. The names are often formed by combining the name of a color and the name of a small animal (Black Rat, Brown Flea, Blue Worm, etc). The names are written on small bronze disks which are attached to their swordbelts.
(3) I made this generalization based on Dany's observations during her inspection of 1.000 Unsullied (3p260). The composition may or may not have been representative, and the composition may differ from time to time, depending on the influx of slaves. It's worth noting that the Ghiscari train and sell their own kind.
(4) They'd also been sold in units of ten (as household guards), but this practice was abandoned because it proved unsound.
(5) Whether they're trained in loose cohort- or tight phalanx-formations (or both) is not made clear.
(6) "In this way, we make certain that there is no weakness left in them.", Kraznys mo Nakloz (3p263). A silver coin is paid to the owner of the child (not necessarily its mother).
(7) Another generalization, based on Dany's observations (3p260). This generalization is supported by a statement by one of the Good Masters of Astapor: "[... and even if we cut the raw boys tomorrow], it would be ten years before they're fit for sale." (3p305).
(8) For information about the Roman legionnaires, please visit: The Military Affairs of Ancient Rome & Roman Art of War in Caesar's Time, The Military Institutions of the Romans (De Re Militari)