I don\'t think devs will be able to create tunels in real time; it would require a tech like Red Faction engine, unless everytime someone need a tunnel, world modelers modify the world.
So another solution might be to have portals (teleport things). They could appear when you say the password, opening a way to the guild. Only great magicians could build portals, and maybe they would have to regularly give them enough energy, I dunno, many things can be done here

Spies could still be useful, as everyone could use the portal with the right password, or maybe even without password, they could pass just after a guild member go through the portal.
So portals could lead everywhere, and powerful guilds could maintain great portals webs so as to be powerful everywhere in the world ^^ (thought great magic would be needed).
Technically, it would just need a portal model, which could be treated as a character for example. Anyway it\'s easier - technically - to add matteer (the portal) than to dig tunnels :p
Now maybe the teleport thing isn\'t a good idea, I don\'t know. It need to be... brainstormed more
