Yeah if they hit your spears in the back, the spears will take heavy damage and probably rout. You have to pay very close attention and adjust the unit's "facing" as soon as you see enemy cav start to move. If your spears are spread out in a long line, it makes them difficult (slow) to turn. Keeping them in a tight block makes changing their facing easier. You can micro-manage facing using the "turn" arrows on the lower left of your screen. If you "g"roup multiple units together and then use the turn function, they will all turn as a single unit. Very useful.