Celts are missing Bracer though, so their Castles are lacking 1 range and attack. That said, they still have the highest DPS of all Castles (regardless of the enemy armour value).Just ran a quick test. For towers this doesn't seem so bad. A tower with 5 Chu Ko Nu only fires 1 additional arrow compared to one with 5 arbs. A castle with 5 CKN fires 3 more arrows than one with 5 arbs, though, which seems like it could get pretty deadly pretty fast.
Obviously what we need is a 2v2 mastapiece with Celt Strongholds Castles filled with Chu Ko Nu killing everything.