I agree something should be done about people not showing up but sometimes there's grey area as to who's allied and who's not, seems like it'd be difficult to police. When all of both EU/NA sides are at least loosely allied, NA players would only be free to defend EU fiefs on sucky ping and vice versa. If there's just a couple people here and there signing up against their allies I don't think it's that big of a deal. I think most people just want to take part in good battles and get tons of xp, they're not trying to exploit the game mechanics.
If you look at your participated sieges, not just recent ones but really far back, while there sometimes a few people from the attacking clan/alliance who signed up for defense and didn't show, there's usually way more people from opposing clans or independent people who don't show. Just looking at DRZ vs. Dhirim for example, Detective Panos style:
1 DRZ on defense who did well.
There's one Nord on defense with a positive kd.
Now the no-shows begin: 1 Bashi, 1 ANT and 2 random.
5 Hospitallers, 1 TKoV, 1 Astralis, 2 green alliance, 1 grey, 1 HRE, 5 random
3 SB, 1 BIA(or Wataga at the time?) and 1 HRE.
So there's 2 DRZ allies (1 bashi, 1 grey)
Six I'd say were probably against them (2 HRE, 1 ANT, 3 SB)
And 17 possibly neutral at that time, random or I dunno (5 Hospitallers, 1 TKoV, 1 Astralis, 2 Green Alliance, 1 BIA(?) and 7 random)
My understanding of who's allied/against/neutral could be a little off but you get the general idea.
Then on the more recent BarLas vs Derchios castle these are the no-shows:
http://c-rpg.net/index.php?page=battlesparticipated#!?page=battledetail&id=37543 Allies: 1 DRZ, 1 Nord, 1 Union
14 Possibly against: 2 ANT, 2 TKoV, 1 Kapikulu, 1 KUTT, 3 Mercs, 1 NH, 1 Occitan, 1 HRE, 2 SB
8 neutral: 5 random, 1 AoW, 1 Caravan Guard, 1 Pillager
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Harold said they recently switched over to hiring system where it looks at your reliability/performance and that's all. Level doesn't matter and you don't have to play before the battle to not get unhired. It's hard to perform well every time, especially with random equipment and spawn killing. People do have control over their reliability though so why not make it so if you don't show for any battle, you're reliability takes a big hit that would take a lot of battles to recover?
That way people would be more careful about signing up for all battles and it would weed out people who don't show on a regular basis. If you're not sure if you can make a npc fief defense, just don't sign up or drop out well before. If you can't make a friends battle they could unhire you beforehand but if you sign up for someone who doesn't know you and you don't show up, they could punish you by leaving you on the roster so your reliability takes a hit. Sounds harsh but it's an option.