I found in plain MTW (not VI) sometimes (rarely) some defenders that were not in castles, but in wood or not. I realised that mostly because after killing all troops in castle the battle still not over and then looking carefully I saw defenders on the map.
When I go for an assault, I shot firstly all available artillery in one point of the wall, that break the wall in one volley. Then I shot to towers, especially catapults. When canon long range available I shot from safe distance almost the entire castle to dust, then I engage with ground forces. I prefer 16 culverins, then withdraw some and bring cav (they reach the target sooner than infantry).
Also, for faster valor increase of the arty, I engage arty crews in battles with enemies. This way, with higher valour, the arty will shot with more accuracy in the next battle (not only sieges).
I found that even if I put in front line of the arty others troops, the defenders will still shot at the arty, not only at the front troops. I prefer to use also the king in assault sieges, because his guards are replenishing for free after the battle. Of course, this is risky to loose the king, but usually he will survive.
It is unclear to me if heavy armoured troops will take less casualties than light troops. Using spread formation will ensure less casualties than close formation.
Because arty accuracy depend on valor, I prefer to use a high star general when assaulting. Otherwise the arty will miss a lot of the shots.
Also I played several castle defences and if you do not have a well defensive unit (as foot knights, varangians or so) to put to hold at the last gate, it is unlikely to win such battles.