Reviewing and playing several defense the tower games, I found that all of them fall in one of four categories, I will try to explain them graphically. Described from left to right, the most notorious difference between all of them is the path that enemy follows, where the goal is, and where you put the defenders.
Fig 1, shows what could be called a “path of rows”, and the most popular game using this scheme is obviously Plant vs Zombies, you put the defenders along the same rows where the enemies will walk, enemies will have to kill the defenders or pass them through to get to the goal.
Fig 2, shows the classical defense the tower path, were yo have this kind of road, Here the enemy must go from the start to the end of the road, which also could be called as the goal, sometimes the roads would have more than one start and end goals. Here, the enemies usually cant kill the defenders because those are positioned outside the roads.
Fig 3, this is also a classical scheme were enemies just follow an straight path from right to left or left to right, trying to reach the “tower”, defenders will be added at the top of the tower and from there they will be attacking. Enemies don’t follow straight rows, they just travel side by side. The tower will have a health bar where you could see how it is taking damage when the enemies reach it and attack it. Several games use point and click to kill the enemies.
Fig 4, In my opinion, this would be the worst schemes of of them all, it is very similar to that described on figure 1 but having only one row.