Knights are pretty much unmounted Cavaliers with slightly better armor.
And siege is in a different category and can't be compared with other units. It may change, but as of now, the only mode where you can have a Siege Workshop and train siege weapons is in Lay Siege. You'll be getting most of your siege through the mercenary markets and perhaps one or two siege engines as rewards for quests.
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"The sky grows lighter my friends. The break of day should cheer the heart. But for us, sunrise brings only bloody war. Make thou ready. Defend my people well." - Prince Erian IV, Dawn of Fantasy