Halfbrick's Colossatron: Massive World Threat is a paid game with several well-integrated free-to-play mechanics, such as dual currency and pay-to-continue. It's one of the most engaging, graphically unique, well designed, and masterfully executed games you can find for $0.99.  

Progression in Colossatron is very straightforward. There are seven countries, each of which consists of four cities. In order to progress from one country to another, a player needs to beat all four cities in a row. The cities are progressively harder, so it’s no easy task. In case a player fails to beat a city, he has to start again from the first city of the country he’s in. 

Colossatron is a blast! Everything from graphics and gameplay to the storyline is polished to perfection.

Gameplay in Colossatron is extremely easy and rewarding - at least during the first levels. The game is built around a giant robot snake (aka the Colossatron) that destroys cities, which are defended by an army. You could think that a player needs to control the Colossatron. Instead, a player’s role is to catch floating power cores and insert them tactically into the rampaging snake robot. Check out the video below to make sense of what I mean.

Combining power cores in the heat of battle is simply a lot of fun! Some combinations are straightforward and logical, for example by inserting three missile launcher power cores, you get one big missile launcher. And, of course, combining three big missile launchers turns into one massive missile launcher.  Other combinations of power cores are more tricky, for example combining a laser beam with a missile launcher creates a flamethrower power core. 

There are both soft and hard currencies in Colossatron. A player earns soft currency based on how much destruction he causes a level. Hard currency is earned through more special occasions, such as destroying all four cities, or by randomly finding a hard currency power core inside a level.

A player mainly sinks soft currency by modifying the the Colossatron between levels. Hard currency, on the other hand, can be used either to unlock and permanently boost power cores or (and more importantly) to instantly revive the Colossatron and thus avoiding having to replay all the previous cities. This pay-to-continue mechanic is most likely the best monetization source in Colossatron as nothing is more frustrating than coming an inch away from beating the country and being forced to replay all the four cities from the beginning...  

There are two main reasons why Colossatron: Massive World Threat remains a niche hit: 

1. It's a paid game in the free-to-play world. With the price tag of $0.99, game installs are limited to a fraction of what could be a free game. It’s a shame as this is a game I believe a lot of people would love.
    
2. Level design: Simply getting rid of the price tag won't turn Colossatron into a successful freemium title due to the structure of the current level design, as well as the overall amount of levels. 28 levels are simply not enough. The little-overshot level design would also quickly result in an overwhelming content treadmill for the team. Maybe streamlining some of the videos between lands and reusing land graphics would help with content creation. But based on how polished the game is, I'm in doubt that the team would compromise production value just to publish more levels. 

Do yourself a favor download Colossatron and check out the launch trailer below:

App Store: http://bit.ly/1fGCUWl

Google Play: http://bit.ly/1fokU2g

Amazon Appstore: http://amzn.to/JKz6Zp