Complex sentence: The attack was so powerful that the knights fighting on foot were immediately overrun and the soldiers, who were holding knights' horses, took flight and the mounted Habsburg army was carried away by the fugitives.
Split into simpler sentences: The attack was so powerful that the knights fighting on foot were immediately overrun. The soldiers, who were holding knights' horses, took flight and that part of the Habsburg army that was still on horseback was carried away by the fugitives.