Device that encases a Stargate in a forcefield to prevent unwanted intruders from arriving on a planet. Similar to Earth's iris, the energy shield encompasses the entire gate. It becomes invisible once polarized. Also like an iris, it can be deactivated by a specific code transmitted through the gate. The energy shield may also be breached using the energy-resistant armor of the Kull warriors.
Earth's Atlantis base in the Pegasus Galaxy -- constructed by the Ancients themselves -- has a forcefield on the Stargate that can be easily activated and deactivated. A Stargate forcefield also encompasses the gate on the planet Erebus, preventing invaders from infiltrating the Jaffa labor camp located there. The gate at Anubis's base on Tartarus is similarly protected.
Orpheus - Thanks to Jackson's ascended knowledge, SG-1 successfully deactivates the Stargate forcefield on Erebus and infiltrates the camp.
Evolution, Part 2 - Jacob/Selmak dresses in a Kull warrior's energy-resistant black armor in order to walk safely through the Stargate forcefield on Tartarus.
First Contact - Atlantis's Stargate forcefield can only hold back an explosion for a few moments, containing a massive amount of energy within it before an explosion tears through the command tower.