The developers often took some inspirations from the popular first person shooter known as Counter-Strike Online (localized as Counter-Strike Nexon: Zombie in North America, Europe and Russia). It is likely that both Smilegate and Nexon did some sort of deal for the content they managed to borrow from them. Here are some of the content that were taken / inspired from Counter-Strike Online:
This S&D add-on uses the same weapon purchase mechanic in CSO, albeit players are limited to only 8 weapons they own (both permanent and temporary). Sadly, Pmang (Former publisher of CF Korea) and Valve never reached an agreement to use this add-on worldwide and thus it was only available in CF Korea (Under Pmang license) and, for a short period of time, CF Japan.
Mutation Mode and Hero Mode were both directly taken from Counter-Strike Online, which were known as Zombie Mod 1-2 and 3 respectively. Both Mutation and Zombie Mod 1 were released with no additional zombies, supplies, health recovery and evolutions. When they released Zombie Mod 2, they did just that, however Smilegate simply added the new features onto the already established mode instead of making a new one.
Then Zombie Mod 3 was released later on, which CrossFire kept the basics intact for the most part when it was released here as Hero Mode. The main differences are that the hero was given a different weapon; Fal-Custom Camo (still with the M203 attachment though) and a mutation vest to give him much more chance to survive.
Later on, CrossFire introduces the Mutant Escape Mode, which is obviously inspired from Zombie Escape in Counter-Strike Online. Unlike the CSO counterpart, this mode has the combination with Parkour Mode, and soldiers has to come to the checkpoint and wait for about 40 seconds for the Golden Portal to be opened to the next stage (as the map has been broken into many parts). Since soldiers in CrossFire don't have to constantly "run and gun" all the time to get to the helicopter, no supporting items is available to slow down the horde, but there are only the appearance of Ammo Boxes placed on each part and specialized character in this mode, like DX Heroine-Puma.
Smilegate started to make their own take on the infection type mode formula; starting with Hero Mode X, followed by Ghosts Vs. Mutants and finally Mutation Knight. Nexon has been doing just that, as they already were. In fact, now they have, along with previous variations, Zombie: The Union, Annihilation, Giant, Darkness and even Zombie Escape!
Fun fact is that, later on, Nexon changed the name of the early variations and at least one of them seems to be inspired from CrossFire, as if they perhaps took inspiration from each other. As Zombie Mod 2 was renamed: Zombie: The Mutation, obviously referencing Mutation Mode, while Zombie Mod 3 was renamed to Zombie: The Hero, which was just ideal and not entirely inspired by the name Hero Mode.
|Dreads / Slugs are based of the regular zombie class, which all have limited range for their attack but their special abilities increase their running speed.|
|Maiden is based of the Light class, which both of them jumps higher than most other mutants/zombies. But their abilities also allow them to turn semi-invisible.|
|Smoke is based of the Psycho class which both spew out black smoke screens to obstruct visibility. One of their arm is replaced; by a drill and a hook respectively.|
|Assassin is technically not based of a zombie class, but rather the zombie grenade item. While acting slightly different, both bombs will disrupt soldiers positions.|
|Enchantress is based of Sting Finger as they jump just as high as the Maiden/Light class, but jump even higher with their special ability, although Sting has limited uses.|
|-||Armored Mutant share a common ability with Deimos, as both will make soldiers drop their weapons, via a roar and a shock wave attacks respectively.|
Aquarium is based of the map: Gallery (which was inspired by a custom map named cs_bloodstrike too), but with different themes. While the layout is slightly different, it still contains keys element from Gallery; mainly the small stair areas on the sides and the vantage points on each corners of the map.
A major difference however, in term of game-play, is that soldiers can actually see through the glass of the Aquariums and notice other soldiers. In Gallery however, the windows are one sided, other side being a texture, therefore unable to see others through them.
This classic map, Crossroad, is based of the map: Ice World (which was inspired by a custom map named fy_iceworld). The layouts are slightly different, but in comparison, Crossroad is also significantly bigger and larger, while also not being entirely a flat terrain (due to the slopes and water in the center).
Major differences between the two maps (beside the theme that is), are the four obstacles. Crossroad's are not all aligned perfectly as Ice World's, which slightly obstruct the view when looking through the center area.
Estate is most definitely based of the Counter-Strike map of the same: Estate. However, it could also been mainly inspired by Counter-Strike Online's rendition of the same map as they were making "zombified" versions of their maps for when players would play them in Zombie-based modes.
CrossFire's Estate takes place within an abandoned empty base with a large outdoor area as Counter-Strike's is an actual estate with front and back courtyards. Similar to Crossroad, this version of the map is significantly bigger/larger as opposed to tight corridors and smaller outdoor areas that the original has.
The core layout remains the same for the most part. There are too many differences, but to mention a few: the sewers is completely lacking, the hallway on the second floor only has one access instead of two and there's an additional balcony at the back of the base.
The modification of the map has many hiding places and shortcut, which makes soldiers easier to hide and move to any bombsite. The main difference that most soldiers don't notice is that the bombsite's names been swapped, unlike the Desert. The reason is probably to avoid the famous saying "Rush B" from CS players.
- A lot of sound effects and weapon models in CF are very similar, if not the same, with CSO - this was the main reason CF is often accused of being CSO clone. Probably SmileGate and Valve reached an agreement for usage of classic CS sound effects in order to make players feel more familiar with CF.