Although the Hypaspist wields a spear, it's not an anti-cavalry unit: it's a melee unit that functions best when adjacent to other friendly units or attacking a district. They also work very well in conjunction with Alexander's Hetairoi - the Hypaspists line up and attack enemy units to weaken them, then the Hetairoi swoop in to deal the killing blow and earn Great General points. Together, they can mop up the forces around enemy cities until the Hypaspists are in position to lay siege to them. Used in this way, a mixed force of Hypaspists and Hetairoi will prove very difficult for other civilizations to fight off in the early stages of the game.
Among the Greeks, a hypaspist (or “shield bearer”) was a lightly-armored soldier who possessed great flexibility on the battlefield. Philip II of Macedon turned them into legends. The Macedonian phalanx, while very nearly impregnable, was most vulnerable at its flanks and in its rear. Though the hetairoi (or “Companions”) could help protect against such attacks, their essential role as shock cavalry meant they were best used in a more offensive role.
Thus, the hypaspistai. With a solid round shield, linen armor, and a short spear, the hypaspistai could rapidly maneuver to protect against attacks from infantry or cavalry. In turn, the hetairoi would protect the hypaspistai, providing progressively flexible flanks around an exceptionally spiky and slow-turning center. Philip’s elevation of the hypaspistai stressed the importance of maintaining the phalanx, and in turn provided an elite unit for young soldiers of Macedon to aspire. His son Alexander the Great would continue Philip’s tradition.