Ok I need a guide through Ultimecia's castle cause frankly I don't know what to do there(meening switching the parties and get all the commands back)!
Secondly where do I find Ultima weapon inside the castle??

Ultima weapon is not inside her castle, you find it in the abandoned research facility where you score the Bahamut GF. Omega weapon is there though. You can find great guides at GameFAQs.com so check it out.

Ok I need a guide through Ultimecia's castle cause frankly I don't know what to do there(meening switching the parties and get all the commands back)!
Secondly where do I find Ultima weapon inside the castle??
you find fuckin' chav weapon here omega

Thanks guys I really appreciate it :)

http://www.gamewinners.com/walkthrough/final_fantasy_8/

enjoy

only omega is in the castle btw, but it is possible to obtain the GF Eden without defeating ultima

Yes, it is but there is no point in waiting until Ultimecia's Castle to draw a very good GF.

Ultima Weapon is not very hard at all anyway. Especially if you have the Lionheart.