ANoxious Blast - I take Noxious blast first becouse it helps me farm minons at low levels.I max it much as other spells becouse Cassiopeia is very active mage and with her abillity power with more spells same rank she gonna deal really big damage.

Miasma - Helps you escape.Good to use after Petrifyng gaze becouse that way enemies wont escape.Even im maxing all spells at same time i keep miasma last in that row of spell point uses becouse it deals moderate damage even at level one.But i take it at lvl 2-before Twin fang becouse it helps me farm minions in general.

Twin Fang - Imporant spell For Cassiopeia-Big damage(no area targeting) almost no coldown when you use it on poisoned enemies.

Runes are tipical mage runes:
Mana.
Ability Power.
Mana Regen.
Magic Penartaion.

Archangel's Staff-Its important to get it fast as you can (After boots OFCOURSE)Anyways.I start getting Abillity power and mana problems are gone.

Rylai's Crystal Scepter-Gives great passive bunch of health(its important to have health at higher levels becouse enemies gonna focous YOU)

Rabadon's Deathcap-After 2 following Abillity power items its time to get some MORE abillity power with Rabadon Deathcap.Its awesome becouse when u combine Rabadon's Deathcap with your last items you gonna have about 300abillity power.Thats awesome!

Zhryona's Hourglass-Not only that it gives abillity power it gives you some armor and great Active:Passive.(allways be ready to activate it)

Rod of Ages-Late game-You need more mana,more health,more abillity power Rod of Ages gives ALL.

-Dont activate your ultimate and Miasma right away(your ultimate should be activated for finish combat becouse it deals much damage and has big coldown,anyway if you use it in begin enemy can flash away,trick you and you will have to wait alot until coldown refreshes.
-Allways use Miasma before Noxious Blast becouse since Miasma is used on enemy Noxious Blast will be easy to use withoun miss.
-Use your Twin Fang only when enemy is poisioned.
-Keeping your enemy in Miasma will keep poisioning your target and gives you great chance to use your Twin Fang alot.

Flash-Helps you keep enemies in range and helps you escaping enemies.
Cleanse-Great in team fights.Removes all negative effects from you and helps you stay active in fight(its imporant to have cleanse ready in team fights becouse enemies will focus you=

I take abillity power in offense.
I dont take Presence of Master in utilly becouse my flash has already decreased coldown and my Cleanse will be used only in team fights(about 5 times in battle)I take Mana and health regen for sticking to your lane and i take Quickness becouse it is very important to move quickly about the field becouse after all field is your territory-take it and use it.

Thank you for giving me views on this build :S
I hope this guide-build will help you on the field!
See ya on Fields of Justice





-dom