An obstacle or entanglement must be broad enough
to not be easily bridged and must be near enough
to be effectively watched at night.
To maintain the obstacle or entanglement
constant care is required. It must be inspected
every night. Resources should be assigned,
permanently, to its repair and improvement.
Every effort must be taken to conceal and protect it;
this is best done by sinking it in hollows or trenches.
Because close and constant observation will be required
loopholes are deemed to be essential.
Build them into your first line.