Your tech advantage, i.e. muskets and tercio vs knights, is not conducive to a good landing. Additionally the AI did not come with overwhelming force. I'm not seeing what you thought was good about this assault.
The muskets I gained during the assault. I both mentioned the lack of tercio (which the AI did have access to and used in the initial assault but never followed up with more) and that it should have committed its forces in bulk earlier. Mainly I was complimenting the positioning, the AI was putting his units in good places for the most part.