So maybe it's not so useful to think of it as just a kind of chess, go, or backgammon. It's a whole new thang, man.
It works in at least 11 dimensions (more on that in future posts).
Tower defense could only have come about as a 'video game' or computer game, because it relies on real-time defense and upgrade of 'towers' and movements of aggressors... TD is a new (a generation old) type of game because it is multidimensional.
. A Risk-type territory grab -- though there are elements of territorial acquisition in some TDs
. RPG (though there may be some role-playing elements involved -- as in you're the general of a base, or you're the commander of the forest guardians, etc.)
. A counting game (checkers, mancala)
Tower defense is a new kind of game. ATARI's Rampart seems to be about the earliest example, and that came out in 1990.Tower defense is not: