The Alliance had begun developing the shield on Ralltiir, in affiliation with the planet's scientists.
Following the Battle of Ralltiir, the scientists were transported to Hoth. Using the salvaged reactor from a scrapped Praetor-class Star Battlecruiser, they were able to provide enough power to raise the shield.
Although the shield was virtually impervious to orbital bombardment (even with the combined firepower of numerous Imperial Star Destroyers and the Star Dreadnought Executor, the Imperials could not breach the shield), it did nothing to protect Echo Base from ground assaults. The Empire therefore opted to attack the base with ground-based assault vehicles.