This is my first attempt at a guide, so any constructive criticism would be welcome. I'm writing this guide because I really enjoy Cassiopeia, partly because she is so hard to play. However I have found her very rewarding, and is able to change the course of team fights.

As a disclaimer this is not the ultimate super competitive high elo guide. I'm only level 27 and never played a ranked game. This is for the people trying to pick her up that play with friends or solo queue. It might work in high elo's but I don't know and that's not what I'm selling it as.

I suppose I should do some pros and cons.

Pros:
High damage output.
Can be very fast.
Twin fang is very scary and fun to use.
Often underestimated.
Great Farmer / Harasser.

Cons:
Squishy.
You will get focused.
Hard abilities to aim.

As a side note people often write how she is less flashy of a mage then say someone like Ryze as a downside. Well let me tell you I think it's an advantage. People don't notice the damage source for a bit so sometimes early/mid game you won't be focused as much.

If you are only skimming, look at the Petrifiying gaze section. Realizing the counter punch trick has helped me out immensely with cass.

I firmly believe in non selfish builds. Therefore you see the abyssal scepter, Will of the Ancients, and Clairvoyance. This is to help the team and by extension yourself. So when deciding what move/action/purchase you are going to make do what's best for your team. Most important thing about this game, and yes sometimes it's best to be selfish.

As for Cassiopeia she is a snake lady. When you think of a snake fighting it pulls up into the air moving around waiting for that opportune moment to jump out and strike. This is the way you need to play. Positioning is the biggest thing with Cass. Once you do it right though you have a surprisingly large burst of speed. Think about it you go 25% faster and slow your opponents by 65% when you land you abilities. You are booking it compared to them for short key points of time where you unload.

WIP

Cass can go mid. However I feel she excels when partnered with a team mate. Think about it you have no CC at all till 6 really but you do TONS of damage with twin fangs. If you get someone who can stun or root you can just unload get yourself a kill, or send them packing home and giving you and your teammate space to effectively farm.

Remember this build is built around living and farming not trying to melt face with twin fangs. 13 creep kills is one player kill. So farm harass and try and get you and your team mate more creep kills while denying your opponents some with Noxious Blast. You can cast a lot of those at the enemy when they come in to last hit and make them go home, or they stay and then you can jump in when you think you can kill them.

The other reason I go noxious blast is that cass is physically weak. So people can drop her quickly. If your opponent has a lot of health regen and decent harass they can back you out. If you went leveling up twin fangs your farm will hurt. However if you go noxious blast you can pretty much safely farm gold and stop creep waves from getting to your turret

This is the ability that makes Cass great in my opinion. Unfortunately it usually is the one ability that takes the most time to get used to. It has bugs, it will stop Wu Kong's spin, but not Kat's. Play her you'll find out how frustrating this ability can be to land. Especially since your opponents could turn their back on you at the last second.

Originally I tried using this as an initiation move. Jump out of the bushes gaze stun people and then sometime my team would jump in and sometimes I would die when the CC wore off, or they had turned their backs. However there does exist a method to almost always ensure cc a majority of their team. Use it as a counter punch. Wait until the opposing team cc's an unfortunate member of your team. Most people dog pile in then and try to kill him quick. Where are they looking when they attack? be behind your cc'd team mate then ult the guys hitting him. You get a lot of their members and the CC on your own member doesn't matter as much because the other team cannot take advantage of his inability to move. Or her's too, women do play this game.

It can also be used to chase someone down, and insure a kill as well as an escape method, or just killing the poor bugger who thought you were easy prey at a quarter life. Really nice for ganks, and catching people under your turret if they try diving to get you.

Primary farm harass spell.

It's a little hard to hit with, but once you practice enough with it it's not too bad. It's on such a low cool down though that even if you do miss it's no biggie most of the time. Just cast it again. It also has exceptional range. If you are in your bushes in your lane you can easily harass/farm anywhere in the lane and stay in the bushes for the most part. Which is a majour strength of Cass'.

Before team fights since it does have such a long range you can get a couple of these in if your careful. But you have to operate at the extreme of the range AND anticipate when they might step forward. If you miss no real big deal anyways, just make sure you stay safe.

It also helps you escape as it speeds you up you can get out of team fights faster, and throw it right behind you to hit that Warwick that's making up ground.

This is your primary damage dealer.

Spam it on the dude you want dead that's poisoned. That's it really it can be used to farm if you really want to push, but for the most part it's just to take one guy down very quickly. It's what makes Gass great sustained damage late game, and horrifying early game if partnered with a singed.

This is a nice spell that sort of fills in the gaps left by the others.

Unlike your other spells it has a longer cooldown so you can't use it as much as a main farming tool, or damage dealer. It does give you line of sight so you don't have to face check bushes, or so you can see where the enemy is going when they chase after you.

Oh did I mention it slows? By a whopping 65% too. Great for chasing and using it in team fights after you ult. They have a hard time running to where they need to be and it applies poison so your twin fangs can go to work. Love this spell very handy.

It can also be cast over walls like into your enemies base so you can harass them rather safely, and accurately.

This ability is what lets you spam your spells earlier without needing too much mana per 5. Or being a little bit more conservative on your spell casting you can be sure your ready for a team fight when it comes.

This is one of my item build I like to go for it helps the team, and gives me survivability.

Rabadon's DeathCap: You need it so your damage scales late game. Just an amazing item really

Rod of Ages: Really good all round survivability item. the health back helps mitigate the no spell vamp.

Will Of the Ancients: Good team item if you have a couple casters as well. Remember most people don't think team first so if you do you can have a bigger impact with their gear also getting the bonuses. Think of a morde with a gun blade and you with this next to him.

Void Staff: The first 100 Magic resist reduce spell damage by 50%. The next hundred reduce it by a further 16% so you get better returns at lower magic resists. Since you've already invested a lot in magic pen putting this in will not reduce the magic resists by much but it will have a big effect.

Abyssal Sceptre: This item I personally love. Not only does it give you a high return for a medium cost item in terms of AP, it also gives you some survivability, as well as team utility. It let's you decide if you want to go defense first if you're having a hard lane against someone like mordekaiser. Or offense first if you have an easy lane.

Mercury's Treads: Good solid boots never a bad way to go.

Other items

Boots of Swiftness: Cass can already move fast when her abilities go off. This makes her faster, and better able to kite. Pretty nice.

Philosopher's StonePhilosophers Stone: A nice early laning item you can get if you are having mana issues and want a little bit more sustain.

Rylai's Crystal sceptre: Health slow ability power good all round. Thing is I think that it doesn't do the 35% on the twin fangs because of it's super short cool down. Not sure though.

Archangel's Staff: Get the tear first then build rod of ages and boots then finish staff. Pretty nice damage boost. It also allows you to be able to spam abilities till the cows come home.

Banshee's Veil: Never forget about this item, MR, health and a great passive. If you aren't getting abyssal sceptre or some other serious magic resist make sure you get this. Unless there are no mages of course.

Guardian Angel: If you are getting focused and CC'd before you can get your ult off get this. When you come back ult straight away. The mechanics of your ult are so strong it might win the team fight. Although you might still be dead anyways.

A note on CDR. I just don't like it with Cass. Your ult is up usually at team fights. Your other cooldowns are fairly low to begin with. Also your main damage dealer does not benefit from it. It has a .5 second cool down whether you are at 40% cool down reduction or 0.

Flash: Probably the best summoner spell in the game always take it.

Only if no one you trust doesn't have it.
Clairvoyance: If someone else you trust has it, leave it for another. This spell is too useful for someone on your team not to have it.

Other good spells

Exhaust: I like this ability a lot, helps get kills secured.

Ghost: Works well with her other abilities to move faster.

Teleport: Useful for getting around.

Ignite: Helps you get that last bit of damage.

Greater Mark of Magic Penetration: This really should be the only rune for a caster here. Nothing better.

Greater Seal of Replenishment: While mana is not a huge issue with Cass she still needs some during the laning phase. This helps ease the growing pains, and late game a bit.

Greater Glyph of Magic Resist: Honestly this part of the guide is where I'm least certain glyph's have a lot of nice bonuses and I'm not sure this is the one I want, but it does add survivability early where you can be real squishy. AP, Mpen, Mana per 5 are all other good options.

Greater Quint of Insight: More magic pen just pile it on. Health or AP would also be good though.

It's really just your basis 9/0/21 build. Just make sure you upgrade the two abilities you want to use.

Not much to this. Noxious blast, Miasma, then spam twin fang and whatever ability that comes up from cool down. Something like this q,w,e,e,e,e,q,e,e,e.

Otherwise in team fights ult then do your combo. That's not the hard part, the hard part is the positioning, and using your ult properly.

I would like to say thanks to other people who have made guides on this web site. I've read a lot and a lot have influenced how I play and made me try some new items. I want to give you credit but I've been playing cass for a long time now and some of the guides I just don't remember who wrote them.

This is a bit more of a survivable build designed to be a little compromising for people who get out of position sometimes, so a beginners build.

Thank you for reading I hope it helps you. Please comment and let me know what you think.