Tower defense games come in many different styles. For example, in the following screenshot of Defense Grid: The Awakening and Unstoppable Gorg, both are top-down isometric style games. However, they are set in different worlds, with different objectives. So, given this, what makes a tower defense game? For one, they are centered on the idea of defending something, whether it is buildings, resources, weapons, and so forth. This is the main mechanism that defines the genre and drives the gameplay. Secondly, most tower defense games require economic and resource management. For example, after each wave of enemies, you may obtain a certain amount of virtual currency that you must then allocate to either the purchase of new defenses (weapons, towers, and so on) or upgrades. Each has their benefits depending on a range of factors such as weak points in your defense as well as the anticipated amount and strength of enemies that will be in the next wave. The number and difficulty of enemies increases after each wave, therefore challenging the player to manage resources and build defenses strategically. The idea is to build up enough resources to upgrade your defenses and to outlast the incoming wave. Sometimes, the player must stop the enemies (or an opponent) from destroying their own base.

In other cases, the player must prevent the enemies from reaching the end, as each enemy that manages to get through cause damage to the player's health bar.

(Top) Defense Grid: The Awakening and (bottom) Unstoppable Gorg

There are many places on the Internet where you can find tower defense games. For example, Kongregate (http://www.kongregate.com/) and Newgrounds (http://www.newgrounds.com/) are examples of sites where a varied array of free tower defense games exist (such as Kingdom Rush or Bloons Tower Defense 5). However, many exist for iOS (App Store) and Android (Play Store), operating systems such as Linux, OSX, and PC (for example, Steam), and consoles (Playstation, Xbox), and so on.