I'm having a hard time seeing how your last paragraph will play out in game. Building a castle cost so much and takes so long its hard for me to envision a scenario where an opposing guild could demoralize the guild who put in the effort to build the castle. If anything, you would think the defenders will be motivated to defend their keep as fiercely as possible.
With that said, if you referring to a hypothetical situation where every time Oranthul built a castle that PRX destroyed it to the point Oranthul gives up on trying to build castles and people quit the guild because it never wins then I can see that happening.
We shall see. I haven't seen anyone really talk about what kind of punishment a guild can effectively deliver in this game to another one. Nobody is really putting pressure on each other in alpha yet, though guilds like OT are regularly focused on.
Traditionally the cycle works like this:
1. AggressorGuild targets DefenderGuild in the game.
2. Aggressor camps them, denies them access to resources, messes up their lands, hits them wherever they can etc etc
3. DefenderGuild puts up a fight but if they lose battles conflict and drama start in the guild (EX: "Why are people farming in such and such a zone when our farms are getting fucked up?? This is bullshit!" vs "You n00bs need to learn the play the game and not getting whipped so easily. That you want me to hold your hand is bullshit!")
4. DefenderGuild starts attriting because of the drama conflict and people getting tired of getting hit all the time.
5. DefenderGuild no longer has the ability to put up a good fight during regularly scheduled sieges.
This has happened in many games for us. Ex: in Asheron's call the Alberon Mindeater monarchy went from 5000 players to 50. In SB on Treachery the Skye alliance went from 3200 to 0. The Ascension alliance in GW2 broke up as soon as they started losing.
What's interesting is that very often the best guilds emerge when you a zerg guild gets smashed by another guild and all the real hardcore PVPers stay together and endure. I'll take one guy who doesn't mind losing for a week over 10 that bug out as soon as they aren't on the winning side.