From what I know about military tactics (in other words what I read off the back of a cereal packet ten years ago) there are six main stages to a successful invasion:[list]
[*][b]1: Infiltration[/b] - Invaders send spies, saboteurs and provocateurs into the target area to recon and weaken defences
[*][b]2: Insertion[/b] - Forward troop move into the area and establish a beachhead for forward operations
[*][b]3: Isolation[/b] - Invading troops attack the lands and transport routes around the local strong points (towns, cities and castles) to cut them off from external aid
[*][b]4: Assault[/b] - Troops attack the strong point directly, breaking the defence and taking hold of them
[*][b]5: Consolidation[/b] - Invaders secure their position, ensuring no counter-assault can retake the strong point and establishing transport routes for their own supplies and reinforcements
[*][b]6: Assimilation[/b] - Invaders convert local populace to their side, either through re-education or by wiping them out and replacing them[/list]
Once the population is on your side, the troops are free to move to the next area and start again. In fact you can start infiltration actions while still awaiting for assimilation to be completed, but you have to be more careful about counter-intelligence operations while working against a potentially hostile native population.

If you simply used Earth magic to burrow straight into the middle of the enemy stronghold you'd find yourself facing a well-armed, well-supplied garrison force who would probably have detected your approach with their own magic-users. If I was them I'd have cleared out the courtyard and started boiling the oil waiting to pour it down on the invaders as soon as they broke soil.