Given the question: Information:  - Tower defense is a subgenre of real - time strategy video games , where the goal is to stop the enemies from reaching a specific point on the map by building a variety of different towers which shoot at them as they pass . Enemies and towers usually have varied abilities , costs , and upgrade prices . When an enemy is defeated , the player earns money or points , which are used to buy or upgrade towers , or upgrade the number of money or points that are earned , or even upgrade the rate at which they upgrade . The choice and positioning of the personal towers is the essential strategy of the game . Many games , such as Flash Element Tower Defense feature enemies that run through a `` maze '' , which allows the player to strategically place towers for optimal effectiveness . However , some versions of the genre force the user to create the maze out of their own towers , such as Desktop Tower Defense . Some versions are a hybrid of these two types , with preset paths that can be modified to some extent by tower placement , or towers that can be modified by path placement .  - Real-time strategy (RTS) is a subgenre of strategy video games where the game does not progress incrementally in turns.  - Strategy video games are a video game genre that focuses on skillful thinking and planning to achieve victory. It emphasizes strategic, tactical, and sometimes logistical challenges. Many games also offer economic challenges and exploration. They are generally categorized into four sub-types, depending on whether the game is turn-based or real-time, and whether the game focuses on strategy or tactics.    What is the relationship between 'tower defense' and 'video game genre'?
The answer is:
instance of