I used to think Castle was underpowered, now that im playing at mid-high plat level, I think he actually is pretty solid for a TEAM pick.
Without a team he is a little underwhelming
One of my favorite plays is on Oregon to Put a Castle blocking off the side closet in Master Bedroom (to the reinforced wall)
Open up the walls of Master Bedroom, and you have made a brutal killzone attackers have to play in as they can no longer just sprint across to the closet for cover.
Close off big window and the window over the stairs and you protect yourself from any attacker holding long angles.
The ONLY change he needs, imo, is frags should NOT break his castles.