The majority of the gameplay involves players tackling a series of increasingly-difficult challenges by drawing lines and filling in as much of the level as possible. The player is vulnerable to attack while drawing — if an enemy monster or projectile collides with the line they have drawn, they will lose a life point, and losing all life points causes failure of the level.

Tarrasque Castle builds a robust metagame around the core Qix-inspired concept. Players have a home base castle in which they can build a variety of structures to generate income and resources. With enough money, they can even purchase "guardian" creatures to protect their castle, which come into play during the player-vs-player component of the game — as well as working through the linear "single-player" campaign, it's also possible for players to attack each others'castles using the same line-drawing gameplay, and having stronger defenses helps make this a more significant challenge for the attacker.