A powerful alien device hidden away outside the main city on Latona. The Sentinel is the sole defense for the Latonans from Goa'uld occupation. It is maintained by a human caretaker who defends it, as well as provides a physical component in initiating it.
The human stands next to the main device, putting hands on either side, generating energy capable of evaporating an entire army of Jaffa and even the Goa'uld ship above.
Force fields surrounding the Sentinel operate on oscillating audio frequencies. There are over one hundred combinations. Pressing various crystals on the panels, in a particular combination, can lower the force field.
The Sentinel - SG-1 learns the rogue N.I.D. group disabled the Sentinel, and pulls two criminals out of prison to go and fix it before Goa'uld forces arrive.