Absolutely, plunder includes prisoners for ransom. Nobles, squires, anyone of any kind of rank or importance is likely to have someone somewhere willing to pay a ransom. Peasants are unlikely to be viable, costing more in food and upkeep than you'd ever get back as a ransom. But anyone of noble blood or high office would be worth taking prisoner.
The noblewoman (presuming she is such) has attacked Sir Cedwyn. That alone removes any oblige noblesse. At this point, taking anyone inside for ransoms would be expected, though they appear to have thoughts on that happening as well and may force you to take them on the point of a sword. Given the attack against his person, Sir Cedwyn would be well justified in simply killing everyone in the castle and putting their heads on pikes as an example to pacify the countryside. It would not do if the Frankish nobles started getting ideas about getting uppity.
Not that Sir Cedwyn leans that way, reputation-wise. But I suspect that, for example, Sir Geoffrey (or most other knights for that matter) would not show Sir Cedwyn's forbearance. Sometimes the ransom is not worth it.
