Teceitos are decently equipped disorderly warbands of Celtic axemen.
The Teceitos (Tek-ee-tos; Axe Soldiers) are a facet of a Celtic army that is not written about as much as their swords and spears, but many Celtic warriors were buried with axes, particularly in Britain and eastern Europe. The axe is a fine weapon, able to do great damage, even through armor. One can actually strike a man in a mail shirt, not damage the mail itself, but kill the wearer by the sheer amount of blunt trauma dealt to the foe's internal organs. As such, armor stands little chance against these men. They wear leather or padded vests and bronze or iron helmets to help protect themselves. They are light enough to move swiftly over the battlefield, allowing them to flank heavy infantry, and strike them from the sides or flanks. Coupled with their axes and javelins, they are especially valuable at breaking up heavy infantry; they make a great asset to chariots particularly, following the chariots into breaches in the enemy heavy infantry, and cutting open the gaps wider.
Historically, Celts were, by the late La Tene, mostly recorded using spears and swords. However, in some regions, Britain and Ireland particularly, the axe was still used, and has been found in numerous burials and hordes, and axes are depicted on Caesar's coins of weapons and armor of defeated Gauls. This should not come as a surprise. The axe is easy to make, cost effective, and can be used to defeat armor or shieldwalls. The Celtic axe is not necessarily the largest of axes, but Celtic metalwork is skilled, so their heads are most likely of good quality, and upkeep would be easy for a Celtic warrior, by himself or with attendants. The head would surely keep a good edge, and even without one, a blunt edged axe can still be used to savagely hack a foe apart without the fear of it breaking like a blunted sword.
Teceitos are flexible troops that make important regional auxiliaries for various factions. With some light armour, a decent shield, two light javelins, an armour-piercing axe, good stamina and decent morale, they have no major weakness. This is a versatile unit that can hold a line at need, but is better at flanking.
The best use of this unit is to destroy heavily armoured infantry, be it Celtic nobles, Roman legionaries, or Hellenic phalanxes. Naturally, most of these units are superior to Teceitos in a one-to-one situation. Therefore, it is best to use Teceitos to flank heavy infantry while pinning them down with other troops.
Similarly, Teceitos can be used against heavy cavalry, with good results. A combination of medium (or heavy, if eligible) spearmen and Teceitos (or other axemen) will defeat any cavalry in this game.
Its versatility, area of recruitment, and overall decent stats make this a unit worth purchasing even if you're not fighting armoured enemies.