The Large Shield Dome is a stronger version of the Small Shield Dome. It costs 5 times the resources and has 5 times the Shield Power. Only one Large Shield Dome and one Small Shield Dome may be built on a planet. Despite their description, and being called a “dome”, domes do not offer any sort of overarching defensive ability. They act as a single unit of fodder.
Some would argue that this makes them not very effective, and that the resources should rather be spent on a Plasma Turret. However, this is not entirely true. Unlike other defenses used as fodder, no ship has has Rapid-Fire against a shield dome. The Large Shield Dome usually prolongs the combat another round, many times being the last object that all the attacking ships target. This could yield the difference between a draw and a defeat for the defender, and thus save the defender’s resources from plunder. If a planet is frequently attacked by Rapid-Fire-heavy fleets (Deathstars or Bombers), the presence of a pair of shield domes serves as a “block” against Rapid-Fire during that combat round.
It takes ships stronger than Light Fighters to damage a Large Shield Dome. Light Fighters are not able to penetrate a Large Shield Dome unless the attacker’s Weapons Technology level is 10 higher and at least twice the defender’s Shielding Technology level. In practice this doesn’t ever happen, thus a Large Shield Dome will never be destroyed if attacked with a fleet of pure Light Fighters.