Math/measurement. If the distance is 100 yards and if you don’t hit the wall after 100 yards, something has gone wrong.
Generally, the enemy will be on watch (or rather, on the listen) for tunnelers, so the further you go, the higher the chance that you’ll be intercepted and killed. Tunneling is slow besides, so you won’t be able to get more than a handful of troops through at a time - perfect for an enemy to get the opportunity to rally their own forces and make that tunnel into a bloody bottleneck for your men. Nonetheless, there are a few examples, such as the Siege of Veii where just that technique worked out.
How do they know how far to dig?
Are there any cases where it is smarter to dig all the way under the wall and then up? (stealth operation anyone?)
Math/measurement. If the distance is 100 yards and if you don’t hit the wall after 100 yards, something has gone wrong.
Generally, the enemy will be on watch (or rather, on the listen) for tunnelers, so the further you go, the higher the chance that you’ll be intercepted and killed. Tunneling is slow besides, so you won’t be able to get more than a handful of troops through at a time - perfect for an enemy to get the opportunity to rally their own forces and make that tunnel into a bloody bottleneck for your men. Nonetheless, there are a few examples, such as the Siege of Veii where just that technique worked out.
„They quickly overwhelmed the Veientines and began a general massacre.“
RUDE