Tower Defence is a genre for both commercial and freeware games, flash-based in particular, on computers, consoles (albeit rarely), and mobile devices. In the genre, players must use money (most of the time) to build defences and survive wave after wave of attackers, and generally survive for as long as possible (or until the end of the game). There are many variations and game modes, such as having waves of stepped up attackers with bonus starting defences and/or money, or gaining no additional money throughout the game after expending the starting pool.
Tower Defence games generally rely on the player spending money on building towers to defend against wave after wave of hostiles. Often, simply defeating these enemies will earn money; in other cases, other, sometimes more creative methods are used. Lives and/or HP play a major part in these games, as they are used (depending on the game) by towers, attackers and objectives alike.
In most games, players have to prevent attackers reaching the end; if they do, the player might lose lives, or the attackers might attack the objective the player was trying to defend. If the player runs out of lives or HP, they lose, and will, in general, have to restart the level.
Enemies are often sent in waves, which organize the attackers into patterns. There may be a timer separating the waves, or the user may be free to send each wave at their leisure. Some games force each wave to begin as soon as the previous is beaten.
Games have numerous methods to gain money. Some give money every wave or period of time (as income), while some games have enemies give money when killed. Some games allow buildings to produce money.