Tower Defense

Tower Defense is a genre built on a single asymmetry: the defender places and upgrades but never directly controls a unit, while the attacker moves on rails toward a goal the defender must not let it reach. The player's entire skill expression happens before the wave starts — where to build, what to build, what to sell — which makes the genre a pure economy-and-positioning puzzle wearing the aesthetics of combat.

The core loop is a currency conversion: kills fund upgrades, upgrades kill more efficiently, and the wave composition escalates to demand a specific counter (armor that shrugs off single-target damage, speed that outruns slow effects, flight that skips ground-only towers). Kingdom Rush and Bloons TD are built almost entirely around telegraphing that counter one wave in advance, so the player is solving a known problem rather than reacting blind. Maze-building variants like the original Flash-era tower defense games add a second axis — the defender can also build the path itself, turning placement into a pathing puzzle against the attacker's shortest-route logic.

Tower defense sits near Auto Battler in that both hand off moment-to-moment combat resolution to the system rather than the player's hands, but the resemblance stops at the board: a tower defense board has a fixed path and asymmetric sides, while an auto battler's board is symmetric and the units themselves carry the strategy through synergy rather than position. Tower defense also shares its escalating-composition logic with Difficulty Curve design more broadly — a good TD's wave list is a difficulty curve made concrete and enumerable, which is part of why the genre translates so cleanly into a spreadsheet-legible puzzle rather than a felt experience.