1. Add penalty for plundering castles of players who are weaker than you, 0-90% less resources depending on difference in power. This way players won't create new accounts just to plunder them or sell large amount of resources that new players get.
2. Add "turns" for sieges. For example, players will have adjustable "clock" where every day you can choose when to have 8 hours turn, two 4 hours turns and four 2 hours turns. During each you can be plundered only 2-5 times, for the rest of the turn it is just siege.
3. Add some protection for newbies in cities. For example, if your castle level is 10 and highest castle level in a city is 15 (5 levels gap), your castle can't be sieged/plundered by stronger players while this castle is in the city.
4. Rebalance "Revenge" option. Right now, if you're a newbie and try to plunder some resources for the first time (or any other case), you'll be pretty much sent to Stone Age by any House through this button, losing all motivation to play. At least make it so other players will have to come to your city (or close to the capital) and only players from attacked city can attack you in revenge.
This way Houses with 200+ members (or less) won't be able to come to you all at once to plunder everything you had without a chance to defend yourself wherever you are. It looks like new accounts is the only way to get resources for majority of players and because of it new players can only start on new servers if they ever want to leave a capital. All numbers are for example. Btw, compare cost of troops in 1.3.4 and in current version.