The Tollan ion cannon is the key weapon in the defense grid of Tollana. A single ion cannon is capable of destroying a Goa'uld mothership in orbit, and many are linked together in a network so that if one cannon is destroyed, the others will locate the position of the attacker and destroy it. Zipacna's forces had attempted to tamper with the cannons, marking them for simultaneous destruction by a mothership, but Lya, of the Nox, used the power of illusion to conceal a single cannon that was able to destroy the mothership and save the planet. When Anubis's advanced technology made at least one mothership impervious to the ion cannon, Tollana became vulnerable. Forced by the threat from the Goa'uld to abandon their isolationist ways, the Tollans offered Earth 38 ion cannons in exchange for trinium. However, their planet was destroyed before the trade was completed.
Cross Reference: Tollana, Tollans
Episode Reference: Pretense, Between Two Fires