When Fieldrunners by Subatomic Studios first launched, AppSpy wasn't even up and running. However, it was one of the very first to be reviewed and with good reason - Fieldrunners came to define the tower defense genre for touch devices, combining an open-layout system and grueling challenges that required players to design their mazes carefully to survive as long as possible.

Almost four years in the making, the sequel finally hits, but with plenty of regular updates to the original and more games in the genre than ever before, is the new game truly a worthy successor?

You would be forgiven for thinking you'd stumbled on to a HD remake - it's an easy mistake to make, but one that's quickly corrected as you delve in to the deep variety of campaign missions on offer. While the changes are light, they are also numerous and the end result is a game that proves it's still the best at what it does.

Your choice as to whether you play the game on easy, medium or hard will affect how quickly you gain access to new towers and power-ups - much of it you've seen before, but you'll also be limited in what you can take with you. Picking the right combination of towers and/or power-ups can be the difference between success or failure and that may be enough for most fans, but hardcore gamers aren't left out of the loop as the harder difficulties have been tweaked to require smarter strategic decisions instead of exploiting AI 're-routing' quirks.

This is a good thing too as the AI has received a significant boost in the intelligence department and are far more prone to 'spreading out', making it harder than ever to take out masses of enemies with poor tower placement. The inclusion of trenches, bridges and tunnels restricts your ability to build complex mazes - something that clearly identified the first title - and also opens up the game to all new asymmetrical map designs that pair perfectly with the different modes on offer.

For the score-junkies out there, you haven't been forgotten either, with 'combos' now affecting your final score and subsequently the amount of coins you can earn for replenishing your stock of power-ups. The combo and 'mega combo' can be triggered by dispatching as many enemies as you can in as short a time as you can, earning extra energy for your 'airstrike' ability in the process.

The game is gorgeous too. Players can zoom in and out on the detailed cartoonish world with ease, and no matter how many enemies are on screen there's little to no noticeable slow-down. It's only a shame the game didn't have a simultaneous iPad release to make use of the larger screen.

Fieldrunners 2 may not represent a huge leap forward for tower defense titles, but fans of the original and those looking to try the genre out for the first time have a lot to look forward to.