Alien Defense is a tower defense game where you protect a central base from escalating alien assault waves. The map is a fixed path that enemies travel from spawn points in the outer ring toward your base at the centre. You build weapon turrets, upgrade existing structures, and manage a resource economy — aliens drop credits when destroyed, which fund further construction. Fail to stop enemies reaching the base and you lose one of three base health points; three hits end the run.
There are five turret types: the Standard Cannon (low cost, moderate single-target damage), the Chain Laser (chains to up to three nearby enemies, excellent against tight groups), the Freeze Emitter (slows enemies within radius, no damage — used to support other turrets), the Mortar (area-of-effect damage, long reload time, ideal against armoured clusters), and the Ion Spike (extremely high single-target DPS, only effective at point-blank range). No single turret type beats all alien categories — a fleet of Cannons shreds small scouts but barely scratches armoured Brutes, which require Mortar splash.
Alien waves scale in size, speed, and armour every five rounds. Round 1 sends five unarmoured Scouts; Round 10 adds armoured Brutes and fast Skimmers in the same wave; Round 20 introduces Boss aliens with enormous HP pools and resistance to specific turret damage types. Preparing for the boss round means saving at least 250 credits before it arrives. Mobile support is full — turret placement and upgrades work via tap, and the UI automatically rearranges for portrait and landscape orientations.
Bends force enemies to slow their apparent movement, but a Freeze Emitter at a bend keeps them in range of flanking Cannons for two to three extra seconds — that extra exposure time is equivalent to adding a full extra Cannon.
Cannons are cheap but their DPS-per-credit degrades rapidly past Round 10 because alien HP outpaces their damage scaling. Transition to Chain Lasers by Round 8 — they cost twice as much but kill three times as effectively against grouped waves.
Boss aliens resist your dominant turret type. Arriving at a boss round with no budget means you cannot counter-build. A Mortar rush is the universal boss counter — deploy two before each boss wave regardless of your primary strategy.