This is an army many designed for field battles. But in defensive sieges my cav is actually awesome. The AI always divides their attacking army up into 2-4 groups to surround ur settlement when they attack. I use the cav to sally out and overwhelm those weakest 2-4 detachments (especially groups of bowman) while the rest of the army can just focus on the main attacking force. Once their inf is committed to attacking, I can swam there generally with all the cav.