Fortress Systems are Sirian star systems that have been granted defences above and beyond those bestowed upon their "normal" counterparts as a reflection of their heightened importance to the Union. Defences are dense, connected, redundant, synchronised, multi-layered and overlapping, forcing a would-be invader through a deadly gauntlet before they can get to the inner ring of defensive positions. Aside from the reinforced garrison present in such systems, a fleet is always placed on Quick Reaction Alert (QRA) to intercept hostile vessels.
The concept was devised by Azosseil Irrazua Nakunn, whom envisaged a "Ring of Steel" shielding the Sirian Union's centres of gravity from harm - or, failing that, make the cost of attacking those centres of gravity so grievous as to cripple an attacker. The defences have been consistently updated with each new innovation in military technology, ensuring that obsolescence is not an issue.
Perhaps ironically, the Fortress Systems have become tourist attractions in the SU, being a source of immense pride both for those under their aegis and visitors from neighbouring systems.