Just like any defense system, this one starts with easier waves and increases the level of difficulty as your stage goes on. One thing not to forget is upgrade your towers which increases both accuracy and RPM of the guns. Every wave of aliens coming from vortexes varies as different kind of aliens with different kind of attributes passes through your lair. From simpleton to heavy armored giant alien.
This app can be downloaded free and you can later on buy upgrade packages so that other guns will be available for use e.g. fire turret and grenade launcher.
Let me know if you want to have the full version.