Tower defense games are a popular genre of video games that involve the player building and upgrading defenses to protect against waves of enemies. These games are typically set up as a series of levels, each with increasing difficulty and complexity.

The basic premise of a tower defense game is to place defensive structures, such as towers or traps, along a predefined path or area where enemies will be advancing. The objective is to prevent the enemies from reaching the end of the path or area, where the player's base or resources are located. As the player progresses through the game, they will earn resources that can be used to upgrade their defenses or purchase new structures.

One of the earliest examples of a tower defense game is the 1990 game "Rampart," which combined elements of strategy and action games. However, the genre really gained popularity in the early 2000s with the release of games such as "Desktop Tower Defense" and "Bloons Tower Defense." Today, tower defense games can be found on a variety of platforms, including mobile devices, consoles, and PC.

Tower defense games are popular for a number of reasons. Firstly, they can be challenging and require strategic thinking to succeed. Players must carefully consider the placement and upgrade of their defenses to ensure that they are effective against the types of enemies they will face. Additionally, many tower defense games include a variety of different types of enemies, each with unique strengths and weaknesses, which keeps the gameplay fresh and interesting.

Another reason tower defense games are popular is their accessibility. Many tower defense games are free to play or available at a low cost, making them an attractive option for casual gamers. Additionally, the simple, intuitive gameplay mechanics make them easy to learn and play, even for those who are not experienced gamers.