To the above, I would add that encircling the enemy line will the cavalry causes the enemy to stop for a few seconds, and reform - without the cavalry attacking. The culverin(s) will do better against these stationary targets. I haven't managed to halt them for more that two or three times, but it is a nice little bonus.

Once the natives get too close, then it is time for a rear-charge. Generals and those pesky priest units are my first priorities...