Siegelight is played with a keyboard and a mouse. You can look round the menus here, but you will need a computer to play.
Monsters pour out of the portals and march on the Siegelight crystal. If it breaks, the siege is lost. Between waves, spend mana on towers along their roads; during a wave, fight beside them and keep them standing. Monsters drop mana when they die: walk over it to pick it up. Every wave is harder than the last.
Each hero builds different towers: walls and ballistae, magic towers, traps, or auras. Walls stop monsters until they break them (heroes can slip through). Wisps fly over everything and hover by the crystal to pelt it: keep something near it that hits flyers (a Ballista, Bolt Tower, Storm Spire or Skyhook, or an Ember Aura beneath them), or go back and fight them yourself. Sappers blow up towers. Every fifth wave brings a Brute; every tenth, the Siege Ogre. Your party shares a defense limit (DU) for the arena.
Larger pixels and fewer effects run faster on slow machines.