Hmm, if you look at how other big servers has had the castle wars it has been so that the winning guild needs to have atleast 10-15 people holding the castle for a certain amount of time. I also think that when its a new battle for the castle current dominating guild should be removed as dominating and they should have to fight in the same way they did last time.
Regarding rewards for holding the castle i'd suggest giving a 5-10% experience bonus. the castle should IMO include an npc that buys your items for a bit more than the usual item NPC buyers as stated earlier. I also think that having private spawns is rather bad. But if you have a private SPAWN it could work. but then limit it to one and put it for players within the range of 200-600 as it will have to suite all players/guilds. and also make it big enough to fit 2 teams of 7-8 people/team or just dont make the spawn at all.
the castle should look just like it sounds. Like a castle. nothing else, but limit the amount of stacks possible so that we dont end up fighting in stacks.
Also, one guild SHOULD be able dominate both castles and it should be visible on the homepage on the website who owns the castles in order to give the dominating guild some credits for taking it. I also think that IF there will be bonuses you should only be able to get 1 bonus even if you have both castles.