As mentioned previously in the thread, a keep really shouldn't be owned by only a handful of players. There's gotta be a set amount of members in a guild to be able to own something as important as a keep. As long as there's at least a fraction of the guildmates around, NPC guards shouldn't even be necessary. However, I am also interested in this "siege tent" idea, alerting players of an incoming siege attempt well beforehand of any actual siege or siege equipment being built. This could the perfect mechanic to combat the ninja-sieging problem if done right. Overall, I think the fewer the NPCs the better. Alert towers are a great NPC-esque feature, however, as long as they're only there for alerting in guild chat... and ringing that beautiful bell!
With regard to walls... As far as guildhalls or whatever for smaller guilds, I'm hesitant on allowing them to have walls of their own. Perhaps SV should allow players to do more activities inside of their houses/guildhalls instead of having players build a bunch of structures outside of them, making people less inclined to build walls around every single thing they own. A lot of the time we can't even see a player's house because it's surrounded by walls. It'd be a lot nicer to instead see the different designs people build with the modular housing system... but then again, most people just build mud brick shit shacks anyway.
Well now I got to thinking that it'd be cool if there were a predesigned guildhall for smaller guilds to build to gather and socialize at instead of a simple modular house. There could even be a small courtyard in the center for the placement of more outside-themed structures like wells, guild priests, etc. For the modular houses I assume they'll be readding, I'm not a big fan of walls at all. Ideally though, I'd like to see players riding in and out of town to conduct their business whatever it may be, and not hermitting in their little hidey-holes all day long talking to their vendors and whatnot.