With each iteration of Pokémon games, Nintendo release two initial versions (which have some slight differences in available Pokémon), then follow this up with an improved, tweaked version. Crystal is the improved version over the 'second generation' games; Gold and Silver.
The typical Pokémon gameplay remains the same. You start out with one Pokémon and travel the world, catching more Pokémon, training them up, and battling your way to beat the Elite Four; the best trainers around. Your secondary aim is to collect them all, but this is only possible by owning other iterations and versions, then trading between them.
Pokémon are found in long grass, caves and water and are encountered randomly. The various species are found in certain areas of the world and have different rarity. Once you enter a battle with them, you engage in a one-on-one fight. Knocking out your opponent gives your creature experience points, or you can choose to capture them to add them to your collection. You do so by throwing a Pokéball which has a random element to your success. Your chances also increase the weaker the Pokémon is, and if it is inflicted with a status (such as Asleep or Paralyzed). You can only carry six Pokémon at all times, so exceeding this limit automatically transfers them to your PC storage facility.
There are plenty of rival trainers to battle with over the course of the game. Defeating them gives you more experience points compared to battling wild Pokémon. Additionally, they give you a cash reward which is used to purchase Pokéballs and healing items from the Marts.
Your Pokémon fall under different categories which modify their strengths and weaknesses. This follows a rock-paper-scissors mechanic e.g. Water beats Fire, Fire beats Grass, Grass beats Water etc. The amount of types in the game, and the fact a Pokémon can be classified under multiple types mean the strategy is much more complex than this.
The moves that a Pokémon learns varies by species. Each move has a type, attack power, accuracy, usage limit, and can have other properties like being able to poison, paralyze, sleep, confuse, burn, freeze and more. Your Pokémon gain more moves as you level them up, but can only know four at a time, so moves must be forgotten. You can also teach your Pokémon moves that can't be gained by levelling up, using items known as TM and HM (technical machines/hidden machines).
There are other types of items available which can heal them or boosts their statistics. You can go to the Pokémon Centre to fully heal your roster for free of charge. If all your roster are defeated, you are sent back to the Centre but with a financial penalty.
In addition to your quest to become the best Trainer, there are a few sub-plots which drive the story. Early on, a thief steals a Pokémon, and this character becomes your rival, battling you multiple times over the course of the game. The Evil organisation, Team Rocket have returned and are up to no good, capturing or stealing Pokémon for their own evil gains, and it's up to you to stop them.
New Features for the Gold/Silver/Crystal generation.
In this generation of games, Nintendo increased the amount of species by 100. There's plenty of completely new species, with additional evolutions and even pre-evolutions of the original 150.
A real-time system reflects the in-game day and time. Pokémon appearances are reflected by the time of day, making the world seem more believable (for example the owl Hoot-Hoot only appears at night since it is nocturnal). There's also certain events like shop sales that only happen on certain days, giving you an incentive to return to the game throughout the week.
Pokémon can now hold items which can be single use items like healing them when their HP falls below a threshold, or restores a status condition when inflicted. Other items are multi-use items which boost certain attacks.
There are some specialised Pokéballs which are better against certain types of Pokémon.
The Pokégear shows the time, map, radio, and phone. The phone allows characters to call the player to offer a rematch, alert them to rare Pokémon, or tell them about rare items they have found and are willing to give.
Shiny Pokémon are an extremely rare occurrence, although the shiny Gyarados forms a small part of the story.
Two brand new types were added which are Steel and Dark.
Pokémon now have genders and can be bred at the Day Care Centre. You can cross-breed certain Pokémon which allows the baby to inherit moves. Some Pokémon have a pre-evolved baby form like Pichu which evolves into Pikachu when happy.
Specifically, the Crystal version of the game allows you to choose a female playable character, Pokémon have animated sprites, and the story has an extra sub-plot involving the cover star Suicune.
After you have finished the main part of the game, you can travel around Kanto which was the region from the first game. There's not that much going on in Kanto, so it's easy to plough through and acquire all the 8 badges.
When you begin the game, it is quite difficult to train your weak Pokémon so you take advantage of healing items, stat modifying moves and status inflicting attacks. However, as you progress through the game, it seems to get progressively easier, because the opponent's levels deviate from yours. Even if you are spending time experimenting with other Pokémon you find along the way, your roster seems overpowered. So near the end of the game, there's plenty of matches where you are achieving one-hit knock-outs which reduces the fun. This problem is specially evident when you reach Kanto. Since you have defeated the Elite Four, story-wise it wouldn't make sense for trainers in Kanto to have stronger Pokémon than them, so you end up fighting weaker Pokémon.
Another problem I had is that you see the old Pokémon that you are familiar with far too much. You are eager to discover the new 100 species, but a lot of them are rare and occur towards the end of the game.
The game was a brilliant sequel to the first generation of games. With a larger world, enhanced graphics and loads of new ideas to make the world more immersive, it is easy to sink hours into the game. Learning about the game's creatures brings a lot of fun and becomes very addictive. It's definitely one of the best games created for the Game Boy Color.