Well, there isn't a lot of damage increase items/cards to be used on the Guardian Stones as there is on the emperium, since it's neutral property and formless race, but it's official defense (40) is pretty high, and looking as it has absolutely no damage reductions besides the normal scripted WoE ones, i'd say it's HP should be HIGHER than that of the current 250m HP Emperium, seeing as Thanatos Sonic Blow, Full Damage Asura Strike, Magic Bolts from an elite weapon/highness staff wielding professor would cause A LOT of damage on the guardian stone. I wouldn't put 400m or more (up to 750m?) as out of the question. Would it take a long time to get the emperium because of it's high HP? Not necessarily, if you have 3 champions doing full asura (10m+ combined damage) per second along with a multitude of meleeing sinX and magic bolting wizards/professors, it's HP will go down FAST.
I say 2 castles is fair. I think ALL WoEs should have at least 2 castles, as the 1 castle in Euro WoE makes it so the entire population concentrates in one area, and that aspect of it makes it a less strategic WoE as it's all about concentrating your attackers/defenders in a sole area rather than trying to juggle to areas.
WoE Drops? Emperium Auroras seem fine. How about some really likeable colors though? As well as there Imperial Valks. I would suggest something else, but we have SO many unreleased imperials/valks/RUCKSACKS. Why not use them for something?? They were created to be used and liked, not to be kept in a small group of people. How about releasing Red/Pink Emperium Aurora? Or any other 2 colors would be fine. As for additional WoE drops, maybe release the well-liked Aquaring rucksack and Firering rucksack? A sort of fire/water themed type thing? That's what I think anyways. Corresponding imperials would also be hot. <3