A sentinel guards against enemies getting in, so an antonym might be a mole, meaning an enemy agent operating "underground."
sentinel
The address of the Sentinel Public Library is: 210 E. Main, Sentinel, 73664 3664
The sentinel watched as I passed by.
The Sentinel - album - was created in 1984.
Gundam Sentinel was created in 1987.
Sentinel - comics - was created in 1965.
The Sleeping Sentinel was created in 1914.
Raytheon Sentinel was created in 2008.
The Sentinel - Pennsylvania - was created in 1861.
Rockhurst Sentinel was created in 1914.
Catholic Sentinel was created in 1870.
The News-Sentinel was created in 1833.