Why didn't they blow the pass....

And of course I have to wonder why Fort Invincible wasn't more invincible in the first place. Why didn't the original builders of Fort Invincible build a series of high stone walls across the narrow pass and have them manned by soldiers with guns and cannons pointing down the pass?

If the enemy have no other pass through the (totally fictional and imaginary) mountains and the hostile Apaches would have to ride for days to get around the mountains if the pass was blocked, why not have the soldiers or whoever built Fort Invincible do a little more work and physically block the pass with walls?

Or maybe plant thorn bushes in the pass and leave space between them small enough for a single soldier to squeeze through with a watering can to water the thorns but not enough space for Apaches to ride their horses through.

Would those walls be washed away by unmentioned seasonal flash floods? The pass certainly seemed dry as dust from what I remember.