I played two games in the tower defense genre (one of my favorite genres): Kingdom Rush and Plants vs. Zombies. The core mechanic of tower defense games is that the player has to utilize defense towers with various abilities to ward off waves of invading attackers.
In Kingdom Rush, each level contains a path with a start and end point which the attackers follow. Depending on the map, there are occasionally multiple start points. Alongside this path, there are prearranged locations for the player to place towers, in hopes of preventing the attackers from reaching the end point. These towers have various abilities including single-target damage, splash damage, slowing capabilities, etc.
In PvZ, each level consists of a grid made up of 5 horizontal lanes of length 9. The attackers (zombies) come from the right side of the grid and the player’s aim is to prevent them from reaching the left side. Towers can be placed in each cell of the grid, and attack down their respective lanes. The zombies walk down whatever lane they spawn in and don’t switch lanes. When a zombie meets a plant in the lane, it will attack the plant.
When these two games are compared, we can see that both games want the player to prevent the attackers from reaching a specific end point through the use of defensive towers. In Kingdom Rush, the path is set so each prearranged tower location has specific advantages and disadvantages. For example, an area-of-effect tower would perform better than a single-target tower on the inside of a turn in the path but a single-target tower would perform better on a straight in the path with its wider range. In PvZ, there are basically 5 “paths” in the form of each of the lanes. Since the paths are straight, the strategy involved in placement is simpler than in Kingdom Rush. Additionally, the distribution of zombies along the 5 lanes is seemingly random and uniform, so the defensive strategy for each of the 5 lanes is universal.
While the strategy involved for tower location is simpler in PvZ, the strategy for tower type is more complex. Since the zombies can destroy the placed towers, a common strategy is for players to create vertical “layers” of tower types with resource generating towers on the leftmost cells and safest from the zombies, followed by damage dealing towers, and lastly, tanky wall towers to block the zombies from destroyed the towers behind it. PvZ also has towers that interact with each other. For example, a pea shooter tower that has a torchwood tower in front (to the right) of it will have the peas it shoots get lit on fire, dealing more damage. Kingdom Rush also has some strategies for tower types interacting with each other, for example placing a slowing tower by a damage tower so enemies stay within the damage tower’s radius for longer, but definitely not as complex as in PvZ.