Tower defense games have come in all shapes, sizes and themes. From defending gardens against zombie attacks to stopping gingers from entering a hospital to defeating robot hordes that march against your base, we've seen just about everything. What we haven't seen is a change to the formula, especially with enemy attack patterns, which are always in straight lines, even when enemies are airborne. Unstoppable Gorg  does away with that pattern, but how is the rest of the game?

The plot goes for a sci-fi theme reminiscent of 1950s black-and-white films. Life in the universe has been found on a distant world beyond Neptune named Planet X, and the people of Earth are eager to meet its inhabitants. To do so, they send out a convoy of scientists accompanied by a military advisor, Captain Adam, and his fiancée, Arielle, who happens to be Miss Universe. Unfortunately, the new aliens — called the Gorg — aren't interested in peace. They view humans as a disease that's spreading through the universe, so they kidnap Arielle and wage war on Earth. It's up to Captain Adam to stop the Gorg and rescue Arielle before the people of Earth are doomed.

The cut scenes show the developers' commitment to the 1950s sci-fi aesthetic. Everything is presented with a rounded TV set and completely in black-and-white. The scenes are filled with lots of stock footage, though a few scenes have been spliced in with modern effects. The space scenes are static, with rockets and UFOs looking like cheap toys or pie tins that are held in the air with visible string. Boost effects are nothing more than sparklers, and the alien "makeup" is comprised of cardboard boxes and rubber masks. The overall effect is cheesy, but it perfectly fits the theme.

The core of the game takes on the basic rules of tower defense. Enemies spawn from spaceships and fly down a predetermined path until they reach the satellite or base that you're trying to protect. The aliens relentlessly attack the satellite or base until it is destroyed, thus ending the level. Your only offense is in the form of towers that can be placed in certain areas of the map. Most of the towers are of the offensive variety, though a few can provide defensive capabilities, such as repairs to damaged towers. Other towers give you more money to buy towers while others give you research points, which can be used to upgrade your towers with increased firing rate and more powerful ordnance.

The big change to the formula comes from orbits. Instead of a grid, you place towers on a set amount of orbits that surround the satellite or base. However, the orbits only have a few spots for the towers. To compensate for the limited space, you can rotate the orbits to give them better placement against the enemy paths. The spots aren't mobile, though, so rotating the orbit affects every tower in the circle.

That orbit mobility helps out in two ways. First, paths may be the same in every level, but the paths change with every wave, making rotation necessary to beat the level. Second, rotation lets you tag enemies as they travel down their paths, a move that compensates for the lack of tower spots on each orbit.

Unstoppable Gorg consists of three different modes. Story mode is the main one, with 21 different levels to defend, and you'll earn new weapons along the way. Foes aren't limited to the Gorg, as brain parasites and robots also enter the fray as the game progresses. Like any tower defense game, different foes react differently to weapons. In a few stages, you'll even have to worry about stray asteroids, which collide with anything in their path. While destruction is key, resource gathering is another thing you'll monitor. Gaining cash is essential for building and upgrading towers, but hitting each level's cash goal also opens up levels in the Challenge mode. Meanwhile, hitting the desired research cap gives you research points, which can be used to upgrade your towers.

What makes people either love or hate the title is the difficulty. At the Normal difficulty level, the game becomes very challenging around the fifth level, and it's maddening after you meet Sereia's brain-sucking aliens. Genre veterans will certainly welcome the challenge, but casual fans and newcomers may not. The title lets you tone down the difficulty level without a progression penalty, but when you do this, the game becomes too easy. Without a middle ground, impatient gamers and those with easily bruised egos will dismiss the title once they hit a roadblock.

Challenge mode lets you replay all of the previously conquered levels, but with a few twists. Some of the changes are as benign as less money at the outset while others are more drastic, like orbits than spin on their own. The same level of difficulty applies, so going on Normal can be very frustrating while Easy feels too easy. The only disappointment is that enemy paths aren't any different than the ones in Story mode, making those stages feel old despite the changes.

Then there's Arcade mode, which acts as the Survival mode. Your tower selection is limited to what you've obtained through Story mode, and the same goes for the enemy forces. If you haven't met the robots in Story mode, they won't appear here, either. Though the gameplay hasn't changed, there are two differences here. First, finance towers can no longer be selected since you earn a preset amount of money at the beginning of each wave. Secondly, the difficulty has been reduced significantly to the point where you don't feel like you're in trouble until after wave 20. Compared to the rest of the game, that change makes it a godsend for those who struggle at the Normal difficulty level.

The only thing that's wrong with this mode is the lack of leaderboard. Dying always provides you with a score based on how many enemies you killed, but the game doesn't bother to post it online, nor does it provide you with a leaderboard to see how everyone else has been doing. There isn't even a local leaderboard to chart your own progress. As a result, the mode feels pointless for anyone who isn't trying to get Steam Achievements.

The sound is done well enough. The voices in the scenes are appropriately bad, with forced bravado from the announcer and anger from the enemy alien forces. Like the cut scenes, the music emulates the score from old sci-fi movies, complete with theremin bits and heavy use of horns. The soundtrack for the cut scenes varies, but the gameplay only seems to play one score. The track is fairly good, but you will notice it after you pass a few levels. The effects are also well done, but the problem is that the default volume drowns out the effects in favor of the music, making for a mostly quiet experience for those expecting lots of loud explosions and gunfire. You can fix this yourself, but it would have been nice if the default settings had taken care of it.

The graphics are simple enough for the genre. The satellites look simple but are distinct enough that you can tell what they are with a quick glance. The enemies aren't that lucky, however, but their various sizes make up for that. The explosions and gunfire are as good as you'd expect, and the backdrops are also rendered well. Their slow, glacial movement fools people into thinking that they're static, but they do the job well enough. You never expect your tower defense titles to be graphically taxing, and you don't expect them to be showcases for your graphics card, but the game doesn't disappoint.

In the end, Unstoppable Gorg is a fun tower defense title. The orbit mechanic provides something fresh for those who've grown tired of the genre's often-used grid system while the old sci-fi aesthetic provides some good laughs. The game thrives on being more challenging than most other games in the genre, so those who don't like a challenge will find the title frustrating. Nevertheless, there are enough good elements here to make it a solid recommendation to both casual and hardcore genre fans.

Score: 7.5/10