The Tower Defence game is a strategy game with the goal to stop the player's enemies reaching a set point on the map. This they do by building a combination of different towers which shoot at the enemy as they pass.
In the Tower Defence game, the towers and the enemies usually have different mixed abilities, upgrade prices and costs. The player earns points or cash when the enemy gets defeated. This they use to upgrade or buy towers or upgrade the total of cash or points that are earned, or if they wish they can apply the points or cash to the rate by which they upgrade.
The core strategy of the game is down to the positioning and choice of the personal towers. It's essential. There are many games the likes of Flash Element Tower Defence which harbour enemies that make a run for it through a "maze."
This enables the player to carefully select the position of their towers for peak effectiveness. Although some versions of this particular variety pressurize the player to create the maze from their own towers, as when playing Desktop Tower Defence.
There are some versions that are a mixture of these two types and there are pre-set pathways which can be adjusted to some extent by the tower placement, or round the other way where the towers can be modified by path positioning.
It is the position strategy of static units that characterises the Tower Defence games, when having to fend off mobile enemy units intent on reaching their end point, from whence they started out.
There is only a set amount of damage a player can handle from the units trying to arrive at their end point, before that particular level is deemed lost. There are some games that depend upon a static route, some that use a free-form type surrounding and some a combination of both, such is the nature of the Tower Defence Game.