Individual Firestorm Redoubts are primarily constructed to protect strategically important objectives from aerial attack and to provide a base for a garrison of troops. With its fully automated batteries of quad Icarus lascannons, any objective protected by a Firestorm Redoubt will not fall to a sudden aerial bombardment and can hold out against a sustained assault until reinforcements arrive while providing shelter for any infantry deployed to protect them against ground-based sorties.
The Firestorm Redoubt combines devastatingly effective armaments with a sturdy bunker complex. Whilst typically equipped to provide anti-aircraft support, some instead sport a range of heavy gatling guns and battle cannons to guard against more typical attacks. In either case, Firestorm Redoubts are usually constructed at regular intervals along with Wall of Martyrs Imperial Defence Lines, providing an interlinking curtain of firepower that is next to impossible for enemy forces to get past.
As said above, it is essentially two Vengenace Batteries plus an Imperial Bunker. Cheaper than the three taken separately, but that's about all that can be said for it.