These tips apply to Pokemon Crystal.  Many of the Gold & Silver Tips work with Pokemon Crystal though.

The Battle Tower in Crystal

This placehas some rules that can make battling much more difficult.  If you have ever played Pokémon Stadium 2, you'll notice the similarites.  The hardest thing about it can be that you have to use pokémon at certain levels.  This means that your pokémon won't have much of an edge in battle, even with type match-ups.  For the lower levels, though, there is an easier way to win.  Use attacks like Dragonrage that inflict a set amount of damage.  Sonicboom will work as well, but it only inflicts 20, while Dragonrage does 40.  Spinarak can learn Sonicboom as an egg move.  Yanma learns Sonicboom at level 19.  Put that Yanma with a female Spinarak. 

Dragonrage can be learned by Charmander and Dratini as an egg move.  At the level 10 and level 20 battles, Dragonrage will KO most opponents easily. 

While Sonicboom and Dragonrage are not allowed in the Petite Cup of Pokémon Stadium 2, there is no restriction on these attacks at the Battle Tower in Pokémon Crystal. ~pokémaniac

Ruins of Alph –

In the first  puzzle room (the one you don't have to surf to) use an escape
rope then renter and a hole will appear on the wall go through it and you
will find 4 items one is a berry i forget the rest then go in another hole
and an unown message will appear and then another hole go through it
andyou'll end up in the ruins of alph

In the second puzzle room (the one when
you just surf and be there no union cave) use flash and a hole will appear in
the wall do what you did inm the first room grab all the items one is a gold
berry and another is a moon stone then every thing else is the same as the
first.

At the chamber with the Kabuto puzzle,a scientist will be behind the puzzle.There he is reading an Unown pattern:Escape.To figure out the pattern use escape rope and renter the chamber.The room will suddenly shake and a secret passage appears on half of the Unown pattern.Make sure you have some room in your pack because there will be four items in the secret passage.As soon as you finish collecting the items,there will be a hole.Go in the hole and you will find an unown message.Finish reading it and go to the crack.You are in the place where you catch the unown.

 In order to get to the secret rooms in the Ruins of Alph to get the hidden Items and the message you'll need these 4 things: An Escape rope for the place that the message is Escape.  Ho-oh in first slot for your pokemon team for the message: Ho-oh.  A water stone for the message: Water.  And Flash for the message: Light. ---Markydd   

  The unown
 
In the back of each puzzle room in the ruins of alph they is a word in unknown writing.
     
In the first puzzle room the word at the back is escape. Use an escape rope and  you will end up outside the first puzzle room then walk back in and the place will start to rumble, then when the rumbling has stopped walk to the back of the room and there will be a hole in the wall, walk into the hole and you will come to a room with items in it. Collect all the items and go to the end of the room. There will be a big hole in the ground, walk into the hole and you will fall through and then you will come to a room and you be surrounded by unknown writing, if you can read unknown writing you will know what it says. At the bottom of the room there is a small hole walk into the hole and you will fall into the ruins of alph. – R. Lee

Celebi - 

This isn't really a tip, but it's just a confirmation. You cannot catch Celebi in the English Version of Crystal. Only in the Japanese version. Hope that helps!  -shiningentei (aka Max)

Odd Egg

Despite which pokemon hatches (this is a randomly determined), the resulting pokemon will have the move Dizzy Punch. Try saving a lot, until Odd Egg hatches.  If you don't like the pokemon, restart, or, you can get a baby pokemon if you want one.  -  David Rubino

---------

Now, When you go to the Pokemon Daycare/Breeding center for the first time,
The Old man will offer you an egg. ( I suggest that before you breed for Baby
Pokemon let the egg hatch! It can disappoint you! Just incase that’s what you
wanted ) This is called the Odd egg. The hatchling will any of the baby
Pokemon Including Tyrouge and has a 50% chance of having Dizzy Punch or being
SHINY this is no Hoax. - Jermaine AKA BeanieBoy

Dragon’s Den

In Crystal once you beat the 8th gym leader you do this quiz in the dragon's
den and if you  win the quiz you'll get the badge. if you come back to the
house in dragon's den  you'll get a Dratini the knows Extremespeed

Mobile Adapter

You cant use the mobile adapter. It is only for the Japanese version.  – Christopher

Catching Pokemon

Here are a couple of tips I found for which pokemon to catch in Pokemon crystal

 1.  Before battling for the badge in Violet City, catch a Geodude on Route 46.  Rock are strong vs. flying pokemon so raise it till it learns rock throw and getting the gym badge is a cinch.
2.  Before batting Bugsy for the badge in Azalea town, catch a Growlithe on Route 36 north of Violet City.  This may be tough because of its Roar attack but if you catch a Gastly first, you can Hypnotize it and catch it.  Raise the Growlithe till it learns Ember (level 9 I think) and beating the bugs is a cinch.
3. Before battling Whitney for the bage at Goldenrod, catch an Abra (with your Gastly's Hypnosis) and then trade in on the 5th floor of the department store for Machop.  Your chances of beating Miltank are very slim unless you have Machop.
4.  In Crystal, Psyduck can be caught on land just north of Goldenrod. Catch one and raise it till it learns confusion.  This is a good pokemon for batting for the Fog badge with Morty the Ghost Pokemon trainer in Ecruteak. - Keith Blumenthal

Legendary Dogs

OK. In Gold/Silver, when you encountered a legendary dog, the normal wild music would play, and you just ignore it, and noticed as soon as you hit run it was one of them. Well, in Crystal, there is a special "Wild Pokemon" music that plays when you run into one of them. Well, this lets you know, its one of the dogs. So no more running from battle, and then finally noticing its one of them. Its probably one of the most innovative idea's Nintendo had.

Other Tips

 You can now get Snubbull without waiting for the phone call on routes 34 and 35.
If you get Dana Lass's number (she's on Rt. 38) she'll give you a Thunderstone.
If you get Gina Picknicker's number (she's on Rt. 34) she'll give you a Leaf Stone.
If you get Todd Camper's number (he's on Rt. 34) he'll call you whenever there's a sale on the roof store of Goldenrod's Department Store. - Samantha 129

Here are some other differences between Crystal & Gold/Silver that Mailen sent us:

A new feature is when you change routes or enter a city. At the bottom of the screen a wooden-like sign will pop up telling you what route or city you are in.

Some Pokémon learn new moves, like Dugtrio knows Tri-Attack and Eevee learns Baton Pass.

Suicune is a major player in the game. After bringing a new item called the Clear Bell (given to you buy the Radio Station manager) to Tin Tower in Ecruteak City, the Wise Trio will tell you all about Suicune and then challenge you to a battle. After you win you'll eventually meet Eusine, a Suicune fanatic, and will continue to see Suicune out in the open while traveling around Johto.

The Unown Puzzles have changed. Now there are messages on the wall written in the Unown language, if you can decipher it you can unlock passage ways to secret items.

Some Pokemon can be found in different places than in G/S.

On your first visit to the Daycare, the old man will give you the Odd Egg that will hatch after you carry it around for awhile.

You can find different Pokémon available at the Goldenrod City and Celadon City Game Centers. The Goldenrod City Department Store has an all-new rooftop store.

Trainers will have different Pokemon to trade.

Other new characters include the PokéSeer, who can tell you where and when you obtained a Pokémon in Crystal Version and how happy it is.

The Move Tutor, who appears outside the Game Corner in Goldenrod City after you've beaten Ash (Red) in Mt. Silver. The Move Tutor can teach very cool abilities, but it's very expensive.

After exchanging numbers with Trainers they may call you to offer a rare stone.

West of Olivine City is a new building exclusive to Pokemon Crystal. It is called the Battle Tower. You can fight trainers one after another, and possibly gain rare items in the process.

After beating Clair for the 8th gym badge, instead of going to the Dragon's Den to get the Dragon Fang and the badge, now you'll have to take a Trainer Challenge. Pass the questions and you'll get the badge. 

A new radio show hosted by Buena can help you gain points and win prizes such as Protein, Iron, and Nuggets. She announces a password during the day, if you go see her at night and tell her the password you'll gain points which can be redeemed for prizes.

Stuff taken out from the English Version

The Goldenrod Pokemon Center has grown into a huge Pokemon Communication Center where you can get information about the Mobile Adapter. (Japanese version)

The most exciting thing people are going to like is the fact that the mysterious Celebi can be captured in Crystal. After receiving the GS Ball in Goldenrod's PCC you can fight Celebi in the Ilex Forest where the shrine is. (Japanese version)

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