Star Citadel
A free web-based version of the 1980 Cinematronics arcade game, Star Castle. Offers a classic mode, designed to play as much like the original Star Castle as possible (i.e., it's really hard!), and a so-called "modern" mode, which is designed to appeal to players who perhaps didn't play the original: it starts off easier, and the difficulty increases in a more progressive fashion.
The game supports all mainstream browsers, as well as running on iOS and Android devices.