No support
I can see it dying quickly because of lack of player participation. I think the time could be better spent. Castle wars in general isn't very fun, especially if you have teams that can stomp. Eventually, nobody will go to Castle wars because it's near impossible to win without collusion or an experienced team.
Also why tf am I always the one that voted differently
It's always about the rewards IMO.
Rewards offered in RS for castle wars were so pointless and rightly belong in the loyalty/vote shops here.
If you could offer unique items that are also tradeable (big selling point here for people to endure the struggle for big profits) as rewards, you would always have people attempting to break through and snag the treasures.
I know this wouldn't be generally accepted, but for example... rare shop offered exclusively through castle wars.
Before you discredit it as "foolish" and "economy-crashing" think of it from every players perspective (IM, HCIM, UIM inclusive)... This would offer a grindable way for them to show off their discipline and dedication because these would certainly be MUCH harder to attain than void, or a blowpipe for example. The items would keep their rarity since they would take almost as much time as a 200M skill to grind out making it like an ultimate achievement to wear.
Of course, not everyone will have the discipline to grind for these, so perhaps offering other lower-tier exclusive cosmetics to the rare CW shop would be nice.
This is only a hypothetical suggestion about how you can entice players to play castle wars and keep coming back, because untradeable ain't gonna do it for most.