Condition Zero is the infamous black sheep of the Counter-Strike franchise, an underwhelming sequel that reviewed poorly and was subsequently succeeded by Counter-Strike: Source half a year later. It is... serviceable, in terms of Counter-Strike games. It's a bit worse for being Counter-Strike, but I don't hate Condition Zero itself, even if it's wildly unambitious (mostly because it was spawned from a failed project that was the opposite).
Condition Zero's signature feature is the campaign. There's a "Tour of Duty" mode where you can recruit AI Bots to fight for you, each with their own stats and weapon preferences. You have a selection of three maps you can play on at a time, each with challenges to complete in order to progress, and once you beat every map in the tier, you move onto the next one. It seems like this was meant to be expanded on later by the community with custom campaigns, but that self-evidently never took off.
Starting with the bot selection, it seems pretty engaging at first, but I think you'll find as you play more you'll quickly turn away from some of the choices due to how the AI works. I ended up using only rifle bots; shotguns are dogshit so shotgun bots are bad by extension, and the problem with SMG bots is that they will full spray without controlling for recoil at range. If they don't land a lucky few hits in that initial burst, they're going to dump their entire mag into the ceiling while the enemy (which seems to all use rifles) picks them off with careful tap shooting. There's also other problems with the AI, like their tendency to stare into walls, spin around in spawn, getting stuck on doors, getting stuck on geometry in general, not look forward while carrying hostages when that's where the enemy is going to be, etc... but it's not frustrating too often. They'll win you rounds more often than you'd expect, and I came to this replay expecting them to not win shit considering my poor memories of their previous performance, but they can certainly do work with the right selection.
The missions themselves are a bit more of a sore point. It's just regular games of Counter-Strike, with some small modifications. The first team to get 2 more wins than their opponent wins. If it's the enemy, they win immediately. If it's your team, you also have to complete all three challenges, which are things such as "Kill 10 enemies" or "Kill 3 enemies with the Night Hawk .40 cal and survive the round" or "Kill a flashbanged enemy". This can spiral out of control quickly; if you get a really shitty weapon on a challenge like the Krieg 550 (it has an accuracy bug in 1.6 and CZ that was never fixed), completing the challenge can be extremely tedious, and after all of that you might run into some unlucky rounds and lose the game, making you start all over again. Losing two rounds to the Terrorists happens a lot easier than you think on the higher difficulties; bot reaction time is extremely variable, and it's essentially CS 1.6, so you're not going to be able to hit anything. Games can last exceedingly long amounts of time, and any charm that was there wears off fast.
By far the biggest issue though is that it's Counter-Strike. Counter-Strike at bots is questionable at best, considering how poor of a combination that is on paper and in practice, but even without that bias, 1.6 is a hilarious clusterfuck of shit balancing and inaccurate weapons (yes, more than GO, look it up) defended by weird nerds who are mad they keep getting one tapped in GO. A lot more luck than you'd think there'd be is involved here, and that luck transfers to the Condition Zero experience, and it can make it pretty frustrating at times considering the low margin for error the game affords you. Losing a single round can be devastating, and while you start with $10000, the rest of the game's eco is normal, so the longer a match goes on, especially when the challenges force you to buy expensive weapons, the more likely it is you get stuck on indefinite deaco. It is 1.6 though, so, at the very least, the Deagle is the only weapon that's actually able to kill enemies.
Oh, one more thing, I forgot about this and didn't abuse it, but I just realized you can cheese the shit out of the bots by just standing in a smoke. That is pretty funny. It's also literally the only way I see people play this game in videos. Maybe this game is just really fucking hard.