Crystal Wars — classic RTS in the browser
Crystal Wars
The map is split by crystals, and crystals are everything: units, buildings, victory. Build a base, send workers to the mines and march your army onto the enemy throne before they do the same to you.

Six unit types with hard counters: soldier beats archer, knight tramples soldier, archer shoots down knight. The mage splashes clumped armies, and the catapult dismantles buildings from a safe distance. Four buildings — base, barracks, academy and an auto-firing tower — plus demolishing your own structures for a half refund when the front moves.
Against you plays a real tactician: the AI phases through opening, boom, mid and late game, scouts your composition and orders counters, sends harassers after your workers, attacks in waves, and retreats behind towers when losing.