Monadius

Monadius is a freeware, open-source space shooter with vector graphics, created as a homage to the Gradius series of side-scrolling games. As in those earlier titles, the player controls a spaceship crossing the screen and shooting down alien forces, while penetrating cave-like structures and carefully avoiding collision with obstacles and the many projectiles from the enemy guns. One of the hallmarks of the series is the menu-based power-up system: some of the destroyed aliens drop energy capsules, that can be collected to move the selection highlight from the menu at the bottom to the desired power-up. There are various types of power-ups available: speed boosts, missiles, satellite guns that surround the ship, double shots, laser weapons and other similar upgrades. Some of them can accumulate, with the menu entry getting crossed out when reaching the maximum allowed. All the levels end with a boss fight, where a large foe has to be defeated by quickly learning its movement and shooting patterns.