Onslaught 2 is a tower defense game that has multiple upgrade paths and combination attacks. The monsters follow a set path to your base and you must use turrets to kill them before they reach their destination.
The object of the game is to prevent the monsters from reaching your base and killing as many of them as you can. Enemies appear in waves with increasing difficulty. To protect your base, you must build turrets to shoot the monsters. There are four types of turrets to build and you can upgrade a turret's damage, range and fire rate. Turrets that have their damage upgraded to the maximum level can be used in powerful combination attacks. There are seven modifier turrets which cause no damage of their own, but enhance the turrets in their proximity. Modifiers are unlocked by reaching a specific number of kills.
Turrets in close proximity that all have their damage upgraded to the maximum level can trigger combination attacks. The types of turrets clustered together and which turret type of the cluster fires first affects which combo attack is used. Different combinations of 2-5 turrets create the different combo attacks.
There are 12 maps which can all be played on Easy, Medium, Hard or Extreme difficulty. Extreme difficulty must be unlocked by scoring 2000 kills on any other difficulty. The game also has a level editor and allows you to load user created levels. There's also a Challenge mode, a Sandbox mode and a Modding feature, all three which must be unlocked through gameplay.