Towerix91 is an incremental tower defense game in the style of PC games from the late 80s and early 90s. The interface, styled after old operating systems, creates the atmosphere of a classic strategy game on a vintage computer. Different types of crystals that drop from enemies are the key to leveling up. The branching upgrade tree offers dozens of options, where each branch splits in two, and the chosen path determines the strategy. More powerful upgrades require rare crystals, which forces you to think through the development of towers and the use of crystals as temporary upgrades during waves. Towers are placed along walls, their abilities are combined, and special skills and upgrades are used to repel the growing waves of enemies. Defeated enemies leave chests with temporary upgrades, gold, or debuffs.