The guardian archetype is broader than the machine.

Cherubim guard the Tree of Life.

The Sphinx guards passage through a riddle.

Talos guards an island.

Artificial watchdogs guard a palace.

boundary + test + permission

The Sphinx:

Prove that you know before I let you pass.

The CAPTCHA:

Prove that you are human before I let you pass.

When machines learn to pass the test, the ancient threshold problem returns in inverted form.