Gem TD, the Warcraft III map where your spare gems become the maze

You pick each gem's spot, and the map picks the gem. The spot you picked to block a corner can turn out to hold the gem you most want to keep, or the worst one. Phone players hit a second catch. The only Android download we found dates from January 2022, the free iPhone app was last updated in June 2023, and the paid iPhone app that's still updated needs iOS 26 or later.

How a round of Gem TD plays

Each player gets a builder and their own area of the map (Gem TD+ changelog, 1.6.0 and 1.6.4). Gem TD+ takes up to 8 players, and its source starts each player on five gem placements and 10 gold.

You see what a gem is only after it lands. It's one of eight types (Amethyst, Aquamarine, Diamond, Emerald, Opal, Ruby, Sapphire and Topaz) in one of six qualities: Chipped, Flawed, Normal, Flawless, Perfect and Great.

Once all five are down you pick one. The map's code then swaps each of the other four for a rock. Rocks don't attack, but they block. Gem TD+ also has slates, a separate family of towers with their own 125-gold Extra Chance option (extra-chance.j), and this page doesn't cover them.

So every round adds one tower and four pieces of wall to your maze, and the wall lands wherever you happened to put gems. Mazing means placing towers so the creeps' route is as long as possible, and our guide to mazing in tower defence covers the shapes.

In Gem TD+, leaks before level 10 only cost time and bounty gold, and any leak from level 10 on loses the game. Gem TD+ had lives until 1.6.0 removed them (changelog, 1.6.0). If you block the route completely, Gem TD+ warns you and teleports the creeps to the next checkpoint (changelog, 1.5.2 and 1.6.7).

The odds behind every gem

Without Extra Chance, the Gem TD+ source sets these odds for each upgrade level. All eight gem types carry the same weight in chance.j, so the type is a flat one in eight. Only quality changes as you upgrade.

Until you buy your first upgrade, and without Extra Chance, every gem you place is Chipped, so early rounds are luck of type alone. Great has a 0% chance at every level. Extra Chance only rolls Chipped to Perfect too, so a Great gem never comes from placing.

Extra Chance bends those odds. Before you place the first gem of a round, you can pay 175 gold to target one type's Perfect gem (Perfect Ruby, say), which raises your odds of getting that type and, at high bonuses, moves most of them to Perfect (extra-chance.j; changelog, 1.5.0).

At 0x, Extra Chance picks the gem for 25 in 80 placements (about 31%). Those are always your target's type and use the odds in the table below. The other placements roll as normal, which makes about 40% overall at 0x. Each bonus step adds 11 in 80 to Extra Chance's share, so from 5x every placement that round is your type. A targeted gem's quality ignores your upgrades and depends on the bonus instead:

Quality of the placements Extra Chance picks. The rest use the upgrade table.

At the 10x cap a targeted gem is either Perfect or Chipped, never anything in between. Extra Chance can hand you a Perfect long before upgrade 8, and Flawed copies of your target stop appearing from 9x.

A hit means a Perfect of your target turns up among that round's five gems (extra-chance.j). After a miss, the bonus grows by one each round you target a Perfect again, up to 10x (the cap since 1.6.0). Switching to a different type's Perfect still counts, because the changelog treats all Perfect targets as one class. A hit at 3x or lower clears the bonus to 0. A hit at 4x or more knocks the stored bonus down by 4, and the next round you target adds one back, so you start 3x below where you were (a hit at 6x leaves you on 3x next time you target).

Skipping Extra Chance for a round lowers it by one (changelog, 1.5.0, 1.5.2 and 1.6.0), so it fades if you stop using it. Since 1.6.9 the -extra command switches Extra Chance off or on at game start (changelog).

These figures come from Gem TD+ 1.6.15, the version maintained today. We haven't checked whether Bryan K's older versions used the same numbers.

Combining gems into special towers

Gem TD+ has three buttons for combining the gems you placed that round: Combine (2x), Combine (4x), and Combine (3x), which lets three of a kind skip a quality (changelog, 1.5.0). The Gem TD+ code shows the cost: once one gem is upgraded, every other gem placed that round becomes a rock, including the partner you used up.

Special towers need a recipe. Epic War's description of the original says you make special towers from gems kept in earlier rounds. In Gem TD+, selection.j also lists Special as a round-end action, allowing at most one special made straight from that round's five gems per round. Once a special is built, you can spend gold to upgrade it (in 1.6.0 the Enchanted Emerald's upgrade fell from 225 to 185 gold, says the changelog).

Swapping gems and switching off the maze

The best gem of a round can land on a dead stretch of the route. In Gem TD+ your builder can Swap the positions of two pieces for 10 mana. The builder starts with full mana, holds at most 10 and regains 1 per level completed (changelog, 1.6.0), so one Swap empties it and the next takes ten levels to refill.

Gem TD+ added a -maze command in 1.6.0 that switches a game between mazing and no-maze modes before the first gem goes down, and no-maze games have their own leaderboard (changelog, 1.6.0 and 1.6.1). The engine struggles with big mazes. "Warcraft III pathing is just bad in complex mazes such as those used in Gem," says the changelog for 1.6.6, from August 2019, after a fix for creeps stuttering.

GemCraft is a different series, and gemtowerdefense.com is a separate remake

GemCraft is a separate series from Game in a Bottle. GemCraft - Frostborn Wrath came out on Steam on 10 January 2020. Its Steam page says you "craft and combine six types of gems" across more than 100 battlefields. It says nothing about random gems or building a maze, and those two things are what define Gem TD. If GemCraft is what you were after, it's on Steam for Windows.

The browser game at gemtowerdefense.com shares the map's full name (the Epic War file is "[BK's]_Gem_Tower_Defense_v3.6.w3x", and gem.j names Gem TD+ "Gem Tower Defense Plus"), but it's Peter Holko's JavaScript rebuild of an old Flash game. It keeps the place-five, keep-one loop, and its recipe list shows 13 special gems of its own: 12 made from three pieces and one from four.

Its rules differ from Gem TD+ in two places. Lives still count there: creeps walk a chain of waypoints, and each one that reaches the end costs a life. Fully encircling a waypoint isn't allowed. The instructions also say you combine two towers of the same type into a higher-quality one, only during placement, and warn that air waves come only every fourth level.

Where to play Gem TD today

The Warcraft III map went through many versions. Version 3.6 on Epic War, by Bryan K (credited as Bryvx UsWest), was submitted on 9 March 2008 for Warcraft III 1.19 to 1.21b. It lists 42 levels and 92 towers, and Epic War lists 41 versions in all.

Two itch.io makers say Gem TD also had a Dota 2 version. Cecrit's page calls its game "heavily inspired by the GemTD custom maps for Dota2 and Warcraft3", and StoneProphet's page calls Gem TD "the popular custom map from WC3 and Dota 2".

Most readers should start with the browser remake, Gem Tower Defense, though it plays by its own rules rather than Gem TD+'s. Its Classic mode runs without an account, and the seasonal modes need you to sign in. Its updates page mentions a mobile start screen and a mobile layout for menus, so it's built to run in a phone browser. On Android it's the one we'd try first, though we haven't tried it there. If you own Warcraft III, play Gem TD+ instead: it's still maintained, and its README says it is compatible with the latest Warcraft III patch.

On iPhone, start with the free GemTD Mobile. Its listing describes the exact loop: gems are created at random, you use one as a tower, and the others turn into stones that block the creeps. It needs iOS 11 or later, is a 1.2 GB download with in-app purchases, and gives you 50 challenge levels, then an endless mode. The alternative is Gem Tower Defense Mobile (£2.99 on the UK App Store, $2.99 on the US one), the one still being updated, if your iPhone meets the iOS catch above. It calls itself "a retro reimagining of the classic GemTD custom-map experience", and every gem you place is both a tower and part of your maze across 50 waves.

Gemaze TD is built around mouse controls, with right-click to remove rocks and middle-click to move the camera, so it suits a computer rather than a phone. StoneProphet's GEMTD drops player mazing altogether, so skip it if the maze is what you miss.

If random gems are the part you miss, Oathstone TD isn't being built for that, because in Maze PvP you'll spend gold on every tower.