When attacking a castle, I split my units. If the opponent gets reinforments, I massacre them, when they try to reach the castle, then I split my units. The ai is terrible at that, so don't expect much losses. Depending on the situation, I send up to 7 units each to 3-4 sides of the castle. As I typically use only yari-ashigaru in most stacks, I rarely attack well defended the smallest castles(so I wait for them to come out). The bigger the castle is, the bigger problems the ai has.
I move my units near the castle (out of range of the archers) and begin to attack at the worst defended part of the castle. I send some units through a gate and some units at the walls(a low wall if possible, never a high wall), but when conquering a part of the castle I use only as much as needed for an effective attack (more would bring more losses). When the area is sure, I send more units in and do whatever makes sense: attacking more units, capture towers, waiting for a counterattack, taking the next gate with the same tactic(up, most times)

At defending a castle I prefer to lure the archers into the castle, so my units can kill them easily. It works in every castle.