Missile Command Deep Dive

Missile Command (1980) is one of the most iconic and successful games of the golden age of arcade games, but it has been ported and remade so many times, that most people aren't familiar with the original version.  Following my deep dive into the game mechanics of the arcade version of Asteroids (1979), I found myself craving a look at the construction of this classic.  My main goal here is to understand how the game components change and work together to produce a fluid experience as level difficulty increases.

Towers

While many of the ports of Missile Command only have one defense tower, the arcade version features three, each with 10 anti-ballistic-missiles (ABM).  The towers are approximately, but not exactly, equally separated from one another.  The center tower is the main workhorse, sporting a firing speed more than twice that of the other two towers, but its 10 ABMs will not always be enough to finish the job.  

When is it better to use the side towers?  If time to the target is the only consideration, then the answer is pretty much never.  In the figure below, I show the parts of the playing area that can be reached more quickly by the side towers than the center tower.  Together they only make up about 6% of the area reachable by ABMs.  Everywhere else on the screen can be reached more quickly by the center tower, and even in the marked regions the center tower will respond in less than 0.3 seconds, more than fast enough for most purposes.  Note that you can't fire missiles to points below the dashed line.

Of course, time to the target is not the only consideration when choosing which tower to use.  In general, you probably want to save your center tower for last-second shots, where response time is critical, and for the the smart bombs, which require very precise timing.  Ordinary missiles can usually be picked off by the side towers without too much consideration for timing because your ABMs create a blast that lasts a full two seconds and at maximum extent covers an area larger than one your towers.

Missiles

Incoming missiles are your primary enemy in the early going, appearing as descending lines with a flashing head.  They can descend on any one of your towers or cities.

Missiles only descend on six targets in a given level, three of which will always be your towers.  It's sometimes a good idea to sacrifice your side towers at the tail end of a level because they will be restocked with missiles in the next round and ignoring them will allow you to focus on defending your cities.  Since only three cities will be targeted in a given level, you're guaranteed to survive any level that you start with four or more cities, including the first one.

Most missiles will start from the top of the screen, but they can also break into multiple pieces during their descent (up to four) or be dropped from bomber planes and satellites.  The beginning of the first level starts with eight missiles and this is the most you'll ever see on the screen at one time.  In fact, no more than eight enemy targets of any kind can appear at one time, including smart bombs, bomber planes, and satellites.

Bomber Planes and Satellites

Starting in level 2, you will see bomber planes and satellites move slowly across the playing area throughout the level.  Satellites take a little over 8 seconds to cross the screen, and bomber planes take a little over 12 seconds.  In the early levels, they serve mostly as an opportunity for bonus points because they fly high and seldom drop missiles, but as the game progresses they fly progressively lower and drop an increasing number of missiles, as many as three at a time.  Only one such vehicle appears on the screen at a time but they will continue appearing, even if you destroy them.

Smart Bombs

After the first five levels, smart bombs begin to appear.  Smart bombs have the ability to move around the expanding blast of your ABMs, making them especially difficult to destroy.  The only reliable methods of destroying them are direct hits or shooting around them until they're forced into a blast.  In later levels, direct shots become the only viable option because you won't have enough to time to fire at them more than once or twice.

In practice, a "direct" hit usually means that you need to hit the smart bomb with the initial blast of your ABM, which only lasts about 1/10 of a second and is about 1/6 the size of a full blast (see image).  It is better to err on the side of firing early because the smart bomb will sometimes travel into the blast, but there is very little room for error.

Scoring

Missile Command uses a fixed scoring system, where a particular object is always worth the same number of points, but with a multiplier that depends on level.  The score multiplier starts at 1x and increases by 1 every two levels, meaning levels 1 and 2 have a 1x multiplier, levels 3 and 4 have 2x, and so on.  After the multiplier reaches 6x, it no longer increases any further.  The table below shows how many points each object is worth with a 1x multiplier.

Point Value for each Object

Object	Points
Missile	25
Smart Bomb	125
Bomber Plane	100
Satellite	100
Surviving Cities (after level)	100
Unused ABMs (after level)	5

Levels

Beyond the overall missile defense concept, the heart of the player experience in Missile Command is the difficulty curve; or rather, the way the game elements change with level to make it more difficult.  Novice players will likely notice the increasing missile speed as the primary factor in increasing the difficulty of each level, and this is certainly important.  The chart below shows how long it takes a vertically oriented missile, starting at the top of the screen, to hit the ground for levels 1 - 15.

So yes, missile speed is a major factor in the difficulty curve, but only for the first five or six levels.  After that, missile speed stays nearly constant from one level to the next.  Does the difficulty remain constant as well?

Hardly.  Starting at level 6, smart bombs begin appearing, and smart bombs are much more difficult to deal with than ordinary missiles.  The chart below shows the total number of missiles and smart bombs in levels 1 - 15.

The number of smart bombs continues to increase up through at least level 15, while the total number of missiles fluctuates up and down without getting consistently higher or lower.  Part of what makes smart bombs difficult to manage is that the finite speed of your ABMs means that you have to account for the distance to the smart bomb when deciding when to fire.  You can perfect the timing for destroying them at close range, but the same timing won't work when trying to destroy a smart bomb at high altitude.

There's one more factor in level difficulty, but I wasn't able to measure it in-game.  From levels 2 through 6, bomber planes and satellites fly ever closer to the ground and drop an increasing number of missiles.  From level 6 onward, bombers/satellites fly at altitudes only 3 - 4 times the minimum height of your crosshairs, which can give you as little as one second to respond to their missiles.  The number of missiles they drop appears to be probabilistic, but it is difficult to quantify because at any given moment, their ability to fire missiles depends on how many things are on the screen and how many missiles have already dropped.  In the higher levels, an undisturbed bomber/satellite appeared capable of dropping as many as four salvos, each of which could contain between one and three missiles.

The Big Picture

The struggle against overwhelming force, and the inevitability of disaster in the face of a nuclear war, are central to the appeal of missile command.  Fear is what will keep the locals in line at the arcades, fear of destruction, fear of this game.  However, it seems as if the designers were faced with some fundamental restrictions in what they could do with it, either because of the limitations of early arcade hardware or some directive from their bosses.  Regardless of the reason, when you're trying to impress the player with increasing difficulty and overwhelming force, it's pretty limiting to only allow eight enemy targets on the screen at one time.  The designers mostly got around this restriction by adjusting missile speed, adding smart bombs, and increasing the role of low-flying bombers/satellites in the later levels.

A novice player would have only seen the first five or six levels, where the challenge is mostly a result of increasing missile speed.  Faster missiles means less time to react, and if you're not quick enough to pick the missiles off individually, there will be some cleverness required to optimize your target selection to destroy as many things as you can with as few missiles as possible.  Beyond level five, however, the developers apparently decided that increasing the speed any further would be an unreasonable demand on the player's reaction time, so the smart bombs and bombers took over.  Consistently destroying smart bombs takes a lot of practice, but is definitely manageable for an experienced player.  I think the last and greatest challenge is keeping the bombers at bay and knocking out any missiles that they manage to get off.  The maximum missile speed can put their missiles out of reach of your ABMs in as little as a second, a window that feels incredibly small when you're trying to manage everything else the game is throwing at you.

That's it for the deep dive, I'll post a full review in my next entry.

NOTE:  A previous version of this article stated that the left tower was faster than the right.  On a double-check, they appear the same and I have removed this passage.