Tower Defense

Tower defense is a game about a route. Creeps walk out of the red square on the left and head for the green one on the right, and your job is to stop them getting there. You do it by spending gold on towers, and the towers do two jobs at once: they shoot, and they take up space. Where you put them decides the route everything walks, which is more of the game than the shooting is.

How to play

Pick a tower from the row above the map, then click or tap an empty tile to build it. Clicking a tower you have already built opens its panel, where you can upgrade it or sell it back for six tenths of everything you have spent on it. On a keyboard, tab to the map once, move with the arrow keys and press Enter or Space.

The route creeps are currently taking is tinted, so you can see what your next tower will do to it before you build. A tile you cannot build on shows a red outline as you hover it, and the reason appears under the map if you click anyway.

You may not wall the route off

There always has to be a way through. A tower that would leave no route at all from the spawn to the goal is refused, and so is one that would seal a creep already on the map into a pocket it cannot walk out of. Everything else is allowed, including making the route four times longer than it needs to be, which is exactly what you want to do.

Creeps do not path around your maze one at a time. The game runs a single search out from the goal across every tile it can reach, and every creep on the map reads the same answer, which is why the route changes for all of them the instant a tower lands. They move on the four compass directions only, so a diagonal gap between two towers is not a gap.

They get in each other's way

Three creeps fit on a tile. A fourth waits at the tile behind until there is room, and if that backs all the way up to the spawn, the rest of the wave waits outside the map rather than being skipped. A long maze with a tight corner in it is therefore worth more than its length: the queue that forms there is a queue standing inside the range of whatever you built next to it.

The three towers

The gun is cheap and quick and shoots one thing at a time, and it is what most of your gold should go on early. The frost tower fires nothing at all — everything inside its ring simply moves slower, which is worth more than damage next to a corner everything has to walk round twice. The mortar takes up four tiles rather than one, lobs its shell at the ground where a creep is standing, and hits everything near where it lands. It is the answer to a swarm and it is nearly useless against one fast runner, because a shell aimed at the ground can be outrun.

Each can be upgraded twice. Every level costs more than the last and gives more damage and a little more range, and the choice between a third gun and a second level on the first one is most of the tactics.

Gold, lives and waves

You start with 110 gold and 20 lives. Every creep killed pays a bounty, clearing a wave pays a bonus that grows as the waves do, and sending a wave early pays one gold for every whole second of the break you give up — which is the lever worth learning, because the break is twenty seconds and the gold compounds. Every creep that reaches the goal costs you lives: one for most of them, two for a tank, five for a boss. At zero the run ends.

Waves come in four kinds, and they arrive as the run goes on: grunts from the start, runners from wave three, tanks from wave five, and swarms from wave eight. Every tenth wave sends a boss in on its own behind everything else. Later waves are bigger, tougher and arrive closer together, all three at once, so a defence that held wave twelve comfortably is usually not the defence that holds wave sixteen.

Every map comes from a seed

The rock, the spawn, the goal and every wave's exact composition come from the short seed shown beside the map, and the same seed always produces the same run. The seed lives in the part of the address after the #, which browsers never send to a server, so Copy link sends someone the exact map you played rather than a description of it. A link always starts a fresh run: your own run is saved between waves in your own browser and picked up again when you come back without one.