Defense Grid

Defense Grid is a straight-up tower defence game. No hybrid mechanisms or other gameplay variants, just towers. For me, it’s the best game in the genre, and it also has an interesting and excellent interface. Let’s look back at everything it did right.

Defense Grid is a 2.5D tower defence game. Enemies come in from pre-determined entry points on the map, head to your power plant to grab your precious power cores, and run towards predetermined exit points. Your job is to place towers to both destroy the enemies, and shape their path around the level.

Defense Grid has a fairly distinctive method of control that is more akin to console games: the mouse cursor for placing towers is always dead centre on the screen. Moving the mouse pans around the level (and thus effectively moves the static cursor around the level). This immediately removes considerations like whether to have edge-scrolling, middle-click to pan, etc. It’s actually a fairly bold interface choice. In a lot of games, like an RTS or a turn-based strategy like XCOM, I think you’d get a bit queasy after a while. Scroll to select a unit, scroll to where you want it to move, scroll left slightly to select the unit’s neighbour, scroll over there to see the enemy status. Most games don’t require you to scroll if you’re selecting something already on-screen, but Defense Grid’s movement scheme requires scrolling to get anywhere. So why does it work?

The answer is that you don’t need that much mouse movement in Defense Grid, other than to pan the view around. You set up your towers, and then you watch the result. At points, you may decide to add a tower, or upgrade one, but they are relatively far inbetween. A lot of the game surrounds judging if your current towers will be enough to destroy the current enemies, and picking the right time to upgrade. In an RTS or similar game, you almost always have two locations used in an action: you first select a unit of yours, and then select a movement destination or target an enemy. In Defense Grid, you have one location to click: the location to place a new tower, or an existing tower you want to upgrade. Less locations to click means most of your input is panning to just observe, and thus it is reasonable to pan by default.

Defense Grid also treats the mouse unusually in the upgrade menu. If you click on a tower, you get a menu up with three options: upgrade, sell, cancel. One thing that’s odd about it is that the cursor is automatically moved to the position of the top option. Moving the user’s mouse cursor around the screen without their input is a risky maneuver, because if they don’t spot where it’s gone, they have to visually hunt around the screen to find it again, and it’s generally an annoying lack of control. The menu also doesn’t let you have free mouse movement, instead it just bumps the mouse cursor between the three menu items, effectively using the mouse to emulate keys.

Defense Grid pulls off this strange behaviour for two reasons. First, when the menu is closed, the cursor returns to its original position, in the centre of the screen. So it’s only a temporary relocation while the menu is showing. With only three menu options, and nothing else to click on, constraining the mouse position to these three items saves time. The fact that the mouse position automatically starts on the top option means that you can upgrade the tower with two clicks (and no mouse movement): one to select the tower and trigger the menu, one to select the (default) upgrade option. So a double left-click on any tower upgrades it.

While the upgrade option is highlighted, you get a nice preview display of the tower’s prospective new radius (see previous picture), which helps you work out if you will reach the alien’s paths or not. Although the 2.5D aspect causes problems here — with big height differences, it’s sometimes not clear where your tower will reach to.

Defense Grid, lets you turn on health bars above all enemy minions. In effect, the state of the game at any time is your tower positions and upgrades — which are under your control — your current credits, the availability of your laser, and the position and health of the enemy minions — which is what you need to monitor. No matter how many explosions are filling in the screen as you rain down destruction, the health bars give you an at-a-glance view of how you’re doing, by being overlaid on top:

Lots of fire and explosions, but one glance at the always-on-top health bars tells me that this wave will be dealt with easily.

Lots of fire and explosions, but one glance at the always-on-top health bars tells me that this wave will be dealt with easily.

The towers are easily distinguished from each other, despite a fairly uniform look. On the screenshots here that may seem unlikely, but each tower has a fairly distinct animation and shot effect which means that after you have been playing for a little while, you never get them mixed up.

I think the main complaint I have with the interface is the incoming enemy display. There are little icons shown to represent the enemies, but there’s no easy obvious relation between the icons and the enemy. Fast enemies get some horizontal lines next to them (like cartoon speed lines) and shielded enemies get a circle, which helps. The main saving grace for the others is that about half of the enemies are almost indistinguishable from a gameplay perspective, so the lack of icon clarity is not so important. But there’s enough room that I think they could fit a small picture of the enemy instead of an abstract icon.

And As For The Game…

Defense Grid is one of my favourite games. Its large number of maps (including the DLC) and several game modes on each mean that I’ve played for over 100 hours but still haven’t got all the available medals. The killer mechanism is the way that your money gains interest if your cores are stolen. This rewards killing enemies early, and building your towers at the last possible moment. It adds a higher-level of strategy when trying for high scores. The sequel is also good, but I didn’t find it added a lot compared to the first game in single-player.


One thought on “Defense Grid

  1. Defense Grid effectively killed the pure Tower Defense genre by being the Platonic ideal of a Tower Defense game. It’s _that_ good.

    One small interface niggle: on Maps with more than one plane (eg: using the actual 3rd dimension for movement), it’s very unclear how towers will affect creeps walking above or below said towers.

    This issue is magnified in the sequel, ironically because map design is a bit better and therefore the problem crops up a bit more often.


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