Successful progression through the waves yields cash with which to buy more towers and upgrades for them; the game also becomes more difficult and the enemies more powerful and tricky as you progress further and further into it. The three difficulties of Easy, Hard, and the ever-popular Medium are available, as is the ability to choose the length of our round, with regular, extended, or endless being the possibilities to be chosen depending on your level of skill and willingness to go the distance.
The real fun in the game is derived from a few sources, but mainly in the freedom of tower placement, which is achieved by tapping and swiping your tower to the exact location that you want. This contrasts greatly with games like Incursion where towers must be build on designated points along the path. This is where the game really shines because not only do you have to consider the type of tower to use in your strategy but also the positions that you place your towers in, since you can actually arrange your towers in a manner that diverts the enemy into different paths of movement and perhaps even right into a trap with a huge number of your gun towers that are ready to mow them down.
You may be happy to know that your towers are also upgradable so that they are equipped to deal with the increasingly varied types of enemy. Helicopters often make an appearance, as do very fast runners that move at a quick pace and slow, large runners that are more difficult to kill. Choosing between machine gun, oil slick, rocket launcher, or electric shock towers is also part of the fun of the game, with each of these towers being more expensive than the last; upgrades are very cheap, though, if a little generic since all that changes is the appearance of your tower and the raising of its power by a small increment.
Fieldrunners from developer Subatomic Studios is by no means a pioneer in its field, but it is a very enjoyable tower defense game with enough levels to keep you occupied and a design that makes it pleasant on the eyes. The rounds are unnecessarily lengthy making you wish there was a hacked version, this makes it a little hard to get stuck right in to the action, but once you've passed the 20th round, you may very well be hooked.