cavalry are strong against missile units.
spearmen are strong against cavalry


Missile units are strong against anyone who they can hit from far range, providing that the units aren't well-armored.

The general rule of thumb is:

Swords beat spears
Spears beat cavalry
Cavalry beat swords

The exception to this is the phalanx (any Greek unit's infantry). If you attack them from the front you will die. So find a unit to pin the phalanx and then flank them (hit them on the sides/rear) with cavalry or whatever.

The most general strategy when lining troops up is the following:

C C I I I I I I I C C
A A A A
G

C = cavalry
I = infantry
A = archers/missile units
G = general

Hope this helps, and happy gaming!