The entrances to those tunnels would only be opened up shortly before the invasion. They're using captive Evokers to make them. With the number of relics floating about, there'll be more than a few there capable of getting through stone, be it through shattering it, modifying it, blowing it up, melting it, using temperature changes to break it, etc. As the Empire has a population numbering in the hundreds of thousands, chances are there's sufficient numbers of potential evokers there to make finding enough possible, as he empire has specialized in capturing and using such evokers. Sep will go into more detail on how they do that. 

The Imperial army striking at this kingdom relies on stealth, which is why they've got countless scouts surrounding the main army and removing any observers. 

One more thing about the Empire: They're not actually evil. They don't kill people for the sake of killing or anything like that. Sure, they might mistreat most Evokers they get hold of, but they still treat them better than the Holy Order of Idris does. And the Empire doesn't discriminate against women, a trait that is unusual in medieval societies.