Base and tower defense: fortresses, turrets and holding the line

The enemy is coming for your walls, and the only question is whether you'll be ready. All five games here are about defense, but each handles it differently: sometimes aim matters most, sometimes trap placement, sometimes the economy behind your army.

The deepest of them is Citadel: Fortress Siege, a medieval real-time strategy in 3D. You start with a lone keep, juggle six resources — wood, stone, gold, food, water and population — place houses, farms and towers on a grid and lay roads back to the castle. In Empire: Build and Defend! you mine diamonds first and then hold your tower against crowds of zombies. Invasion of Monsters: Don't Let the Monsters Pass! is all about placing cannons, mines and barricades in front of your shelter.

If you'd rather do the fighting yourself, try Defending the Planet: as a starship captain you fend off asteroids and aliens while the difficulty climbs smoothly. Zombie City lets you hold a position against hordes with a big pick of weapons. It's a small collection, but it covers both careful planning and straight shooting. Free to play, nothing to install, no account required.