It is a simple matter to play the single-player campaign from the original Quake II using the Ground Zero settings. As well as new weapons and enemies, Ground Zero also alters the behaviour of many of the original Quake II enemies, making them more challenging in some cases. So, this is a way to make the original Quake II campaign more challenging. No modding is necessary to do this; all you'll need is to have both Quake II and Ground Zero installed, and then use a few quick console commands.

1. Launch Ground Zero via Steam.

2. Begin a game of Ground Zero on whatever difficulty setting you wish to use to play the original Quake II campaign. If you want Hard+ difficulty setting, then start a game on any difficulty setting, then change it to Hard+ by opening the console and enter the command "skill 3" (without quote marks).

3. Open the console and enter the command "map base1". This will cause you to spawn on the first level of the original Quake II campaign (Outer Base). However, all of the enemies will now behave as if you were playing Ground Zero.

That's it. However, there are two minor problems - see the "Game crashes" section below.

Many of the original enemies from Quake II have had their behaviour altered in Ground Zero, making them more challenging in some case. A quick summary:

- Enforcers attack more quickly, due to their reload animation happening after firing, instead of before firing (which was also the case in The Reckoning).
- Gunners may continue to fire up to three grenade volleys if the player disappears from their sight.
- Tanks and Tank Commanders will usually follow-up their attack sequence with an additional rocket barrage which fires with less delay than normal, even if the player is no longer visible.
- Iron Maidens, if they lose sight of the player, will often continue to fire rockets towards the last spot they saw you.
- Parasites will advance more consistently rather than stopping every few seconds.
- Flyers and Technicians may strafe sideways when firing.
- Medics will revive fallen Strogg much more quickly, whereas in the original game they would often take some time even to approach the corpse.
- Hornets use "predictive fire" with their rocket salvo, aiming some of them towards the position the player is moving towards, so they cannot easily be dodged by strafing (It's odd that this was changed for Ground Zero, because there are no Hornets in the Ground Zero campaign, but whatever)

Furthermore, some enemies tend to sidestep to try to dodge the projectiles fired by some of your weapons, especially the Hyperblaster. Enemies on a higher level of elevation may also sometimes jump down to approach you.

The five new weapons from Ground Zero will not be found in the original Quake II campaign. However, if you wish, you can use the console again to simply give yourself the weapons. The commands are as follows:

give etf rifle
give plasma beam
give prox launcher
give chainfist

Tesla Mines and ammo for the ETF Rifle and Prox can be obtained as follows (replace ** with the number you wish to obtain):

give tesla **
give flechettes **
give prox **

If you play the single-player Quake II campaign through Ground Zero, there are two points in the game at which the game will apparently crash:

1. Pressing the switch in the Big Gun.
2. Shooting the secret button that disables the laser trap in the Lower Palace.

I am not aware of a workaround for these two bugs, so I would recommend simply not touching these two things. When you have cleared out the Big Gun of enemies, don't press the button but instead return to the locked door at the beginning and use the console command "noclip" to move through it. As for the secret button in the Lower Palace that disables the laser trap, just don't shoot it.

Incidentally, you can do the same thing to play the single-player Quake II campaign through The Reckoning instead of Ground Zero. It's done in the same way - load up The Reckoning, choose your difficulty setting, and then "map base1" in the console.

However, as far as I can tell, the only thing this really changes is the Enforcers will fire more quickly, and all of the Brains in the campaign will be replaced with the more dangerous Beta Class Brains. But you could give yourself the Phalanx Particle Cannon, Ion Ripper and some Traps if you wanted to.