well, the AI cavalry is programmed to avoid spikes at all cost. There are (probably ahistorical) ways to "lure" AI cavalry into them (placing a unit halfway on the spikes, halfway in front of them, pulling the unit back before cavalry impact).

on another note, spikes are useful to protect the archers from frontal cavalry charge. they can sit behind the spikes and shoot away in straight trajectory (rather than over the heads of spearmen, for example), while spears protect flanks.

infantry will still be able to go through the spikes, but infantry is slower and more advanced longbows can hold their ground against infantry anyway. alternatively, pull the longbows back before infantry contact.