Pikemen becomes available during the Feudal era. They are a very useful unit, being much stronger and faster than Swordsmen.
Per-unit, Feudal units outperform even artillery units of the Classical Age so upgrading will keep the numbers more granular and easy to see, making them readily manageable (see footnote for details).
These units fill the primary role of the Swordsmen ranks from the Classical Age, being much the same in upkeep and cost, scaled up to the Feudal level. Their variants similarly perform below par and at a cost-equalised basis, most armies will employ a greater number of Pikes than a halved number of specialists unless training time/investment is a hassle.
Pikemen similarly work well with a large cadre of Longbowmen and Mangonel.
They can be improved with Pikeman Training.
They have a raider variant; faster, stronger, but less survivable variant called Pollaxers, and a defensive variant called Halberdiers.
Barbarians use a variant called Berserkers. They become obsolete in the Imperial Age.
It is possible a greater number of Classical units could be trained at more efficient rates, but this would often be impractical and confusing. In addition, as the ages increase this tactic becomes increasingly less viable as rounding becomes more accurate.