Why Castle Federation phases out heavy armour

Were Stewart's ships protected by extremely thick armour?

The example is limited to the original Avalon in Castle Federation, which is being retired as armour is phased out because it cannot stop the available weapons.

The heavily armoured ship remembered in the question is identified as the original Avalon in Castle Federation. That ship is being retired, and armour is being phased out because it can no longer protect ships against the setting's weapons. The example gives a specific design consequence of the powerful weapons discussed earlier.