Sword is an odd comparison because it is a much more efficient vessel than the castellan, point for point. Infidel is a bit more lop sided admittedly, although saying that tau torpedoes are 100% than IN is absolute crap. The shrinking factor has come into play multiple times in games I have played, rendering crippling torpedo salvos into mere annoyances.
Second, if also competes against the new defender. Give me 9 defenders versus 10 castellan's and you would have a one sided slaughter

And the defender's would feast on the corpses of the more "advanced" ships.
The defender outguns it and is significantly harder to kill....for 5 points. True it does have it's advantages, but nothing comparable to the huge firepower and resilience that the defender gains. For 5pts. And the castellan is much more reliant on SO than it's brother. Miss an SO and that castellan is pegging away with its puny batteries. Miss an SO and your defender still has the battery output comparable to a lunar cruiser.
To compound that, they are both available without restrictions in the same fleet lists.
It's good that the defender will see more play; it's just a shame it went from being pretty bad to quite good whilst the castellan is mired in mediocrity, in comparison if nowhere else. Maybe I can find some defenders on ebay...