What makes you think a tunnel would be safe? Just need to re-purpose this little guy:
https://blog.sintef.com/digital-en/inachus-project-robot-sea...
a network of tunnels and sensors is easier to defend than a open space exposed position where drones can see you from miles away, I guess that makes tunnels better for defense
the operator of those would get hit by an aerial drone from one of the other tunnel exits