Star Citadel is a version of the not terribly well known arcade game Star Castle. Star Castle is a vector shooter released by Cinematronics in 1980 and is perhaps best known for providing the inspiration behind Yars' Revenge, developed by Howard Scott Warshaw for the Atari 2600.
Star Citadel includes a classic/retro game mode, that is as faithful as possible to the original Star Castle, and an updated modern game mode. The latter includes easier/more responsive controls, power-ups, and more challenge in the form of multi-hit shields and mines.
As a big fan of retro shoot-em-ups of all varieties I hope you enjoy this!