This post is from the now defunct website “A GameDev Plays…”, copied here for posterity
Despite being nearly a decade old, I still enjoy playing Defense Grid, a tower defense game that largely defines the genre for me. Here is a video of me just this week completing the final story mission without losing a core - it only took me several years!
The basic idea in tower defense games has the player placing various structures that automatically attack targets within range around a map which has waves of enemies heading towards something important. And with that description, you now know almost everything required to play Defense Grid. When I think of this genre, I think of this game - it defines tower defense for me. Partly because it is so good and repayable that it still sits on my hard drive years after installation.
In Defense Grid the enemies are aliens and the macguffin you are defending are power cores. These cores are used to run the towers so when they are all gone, the level is lost. However, until then it is alien bashing time. There is a basic story behind the game, but this can be largely ignored. Reactivate defences, defeat aliens, blah blah. The game is just thematic enough that the plot doesn’t detract from the game, but can’t claim it boosts it either. Here, gameplay is all important. There are many types of tower each with different uses: some require line-of-sight; some are good against shields; some against armour. There are anti-aircraft missile silos (useless against ground targets); towers that slow nearby enemies; and, towers that illuminate otherwise invisible enemies. All of these can be upgraded and have different resource costs. The opposition consists of aliens of various types and characteristics, marching in predefined waves towards the cores. The game is one of deciding how to best place the towers for maximum effect against these waves. Sometimes the player has the time and resources to plan carefully; but often they are desperately building up as aliens threaten to overwhelm the defense. Survive all the waves with at least a single core at the end and you are victorious.
A key design decision in the game is that towers block enemies’ movement. Thus a very successful tactic is to channel the aliens with careful tower placement into mazes and traps. It can be strangely satisfying to build an engine of destruction and then leave it to run - funnelling hopeless enemies into prepared killing grounds (who cares, they are just aliens right). There is also an orbital laser for dealing with aliens that manage to break through the defences. Although, this can only be activated a few times per level (it has a long cool down timer), so using it needs to be carefully considered. Mostly this game is about deciding the best placement for towers - as it should be in tower defense!
This game does many things right so the player can focus on the sheer enjoyment of successfully building a death-maze. There seems to be no, or extremely little, use of randomness. The maps are hand-designed and always the same for a level. The number and constituents of the alien waves is fixed - play a second time and you will face exactly the same enemies in the same order. The path taken by aliens is deterministic (the shorted route to the cores) and the damage done by your towers appears to be fixed. In other games this could lead to predictability, but here the complex array of options and the size of the maps means each different plan quickly diverges from previous encounters. The lack of RNG just means the player knows that success or failure is entirely up to them.
There are also a number of extra challenges in Defense Grid. The story has 20 missions and takes several hours to play through. However, the story levels comprise the easy mode! Each level has various additional objective modes to make it more interesting or harder - complete the level with more enemies or without using a certain type of tower, etc. There is also DLC to add extra missions/maps (I have not played these). I particularly like that the default story is the easiest with the challenges on top - players can choose their difficultly without stigma of decreasing difficulty to easy-mode.
To emphasise the tactical nature of the game, the mission does not start until the first tower is placed. Thus the player can have long as desired to create their initial plans, before watching them fall apart in the heat of battle. This initial time is particularly useful given the only negative point - there is no zoom-out. Most levels are large enough that they can’t be seen in their entirely at once, instead the player needs to pan around to get a complete view. There is a zoom-in control, but this serves no tactical purpose. It just allows towers and aliens to be inspected in great detail (the game has good graphics considering its age). It is also possible to speed up time; handy as at normal speed some of the levels can take over half an hour.
Moving the view around the map is implemented in an unusual way. The targeting reticule (for placing towers) is fixed at the centre of the screen. Moving the mouse pans the entire screen. This means your view is always changing, even when placing neighbouring towers. It feels like the game equivalent of jiggling your foot or tapping fingers. Luckily though the player does not need to move the mouse too much, it is just a matter of choosing where to place the towers and then watching the resulting mayhem. I also like that the developer has minimised the use of numbers. Tower and enemy stats are displayed as graphs and upgrade level by colour. Experience has shown that less numbers are better as far increasing player involvement, and it works fine here.
Highly recommended for all fans of tower defense. Defense Grid plays strongly to the genre tropes with well-designed, repayable and enjoyable gameplay.