http://lifeisfeudal.gamepedia.com/GuildThe way I understand it a kingdom is just the leader of an alliance of guilds. According to this the only difference is they get special titles and their political standings automatically apply to all of their vassal counties.
http://lifeisfeudal.gamepedia.com/ClaimOn the main website is mentions town claim increases with monument level, but it doesn't mention that here. I don't know if the 70m radius is what towns will start at or what they will max out at. I mapped out a 70m radius (35 cells). It's pretty big.
Keep in mind that when at war, battles lower the level of a monument. Once a monument reaches level 1 a siege can be started. Town claims can only be attacked in a siege. If you built a castle outside of the town claim, players could attack it at any point during a war. Realm claims are just a way to control who can have a personal claim on that land.
I may be mistaken, but I am pretty sure a monument can be destroyed during a siege, taking that land away from the guild/country. I think it would be wise to have it safely protected in the middle of a castle.