Ipsen's Castle is marked on the map, it is on the northern region. of the Forgotten Continent. You can press select while in the airship, move the cursor and hover it over Ipsen's Castle on the world map. Press X and choose to go to Ipsen's Castle. The airship will take you there automatically! Land on the green patch just outside of Ipsen's Castle and then head inside. A short scene plays, Amarant decides to ditch the party and go in by himself, his reasoning is that working alone is more efficient than working in a team. Whatever... Whether you choose to let him go or not, he's going. You will then have the chance to set up your party. My party consisted of Zidane, Freya, Vivi and Eiko.
Before going inside, go into your menu and equip each of your characters with the weakest weapons in your inventory, the weaker the better! You might still have all of your old weapons, if not you can find the weakest weapons possible within Ipsen's Castle. When you are finished climb up the steps and head through the entrance. Move over to the left and search by the pillar for a chest containing a Dagger. Search over to the right by the pillars to find another chest, this one contains the Aquarius Stellazzio. Continue onwards, talk to Kumool to save your game, you may have a letter for him as well. He also has a Mogshop, note that he sells Vaccines. Stock up on them because they are quite hard to come by.
Head through the pillars to the left and follow the path to the end, open the chest to obtain Cat's Claws. Approach to the pole to the right and slide down. Make your way to the ladder on the top left side of this room. Climb up to the top and follow the path to the door, proceed through it. Make your way into the next room, continue climbing the ladder until a '?' appears, then choose to jump off of the ladder to the left. Open the chest containing the Javelin, go back to the ladder and this time select to jump off to the right. Follow the path and open the chest at the end to obtain a Broadsword. Return to the ladder, climb down to the bottom. Follow the path up the stairs and jump onto the ladder. While climbing, choose to jump off when the '?' appears. Follow to the end of this past and open the chest to retrieve a Rod.
Return to the ladder, climb up to the very top, Zidane will hop off onto the balcony. Proceed through here, move down the steps and examine the wall to the right. Press X when the '?' appears. First select Examine, then Push, then Pound, then Think, Push again, Try something drastic, and finally Rest. Head to the right, open the chest to find a Barette. Return to the last room, head back up the steps and step onto the platform. In this room you will find Amarant, once he is gone approach the wall and remove the four mirrors. You will obtain the Wind Mirror, Water Mirror, Fire Mirror and the Earth Mirror. Attempt to leave the room, your party members will have a short chat, suddenly a boss battle ensues.
This is an easy battle. He doesn't deal much damage, he uses some physical attacks and either Thundaga or Blizzaga. He will be down quite easily if your characters know some decent abilities. If Vivi knows Flare cast that, if Eiko has learnt Holy or can summon Madeen use that. Have Freya and Zidane just attack normally, Zidane should steal from him as with most of the bosses. Stealing is made much easier if Zidane has the Master Thief support ability equipped.
Just continue pummeling Taharka with spells and attacks, he will be down in no time.
Leave this room, head all the way back to the floor with Kumool the moogle. The trap doors are activated when you reach the top. Move around the floor the top left section, you should be able to see the trap door slightly, stand over it and Zidane will fall down to the level below. Open the chest to retrieve Maiden Prayer. Climb back up the pole, when moving around this area watch out for more trap doors, none of the other ones lead to treasure. Exit this area and go back to the first room. Take the stairs in beneath the balcony, follow the path until you reach the room with the pole. Run to the left and open the chest in the corner to the left to find an Air Racket.
Stand on the platform with the pole and it will take you down to the next level. Open the chest to the left to retrieve a Golem's Flute. Move the two pots between the three platforms in a counter clockwise direction. When you hear the voice, place them on the two platforms on both sides of the sword. Pick up the pot to receive Ancient Aroma. Now it's time to leave Ipsen's Castle, so head back to the entrance. Before leaving you can climb the pole in the room with the moogle, open the two chests on the platform to find a Fork and a Mage Staff.
When Zidane and the others arrive back outside, Dagger informs him that Amarant hasn't come out of the castle yet. Zidane chooses to head inside to look for him, try to avoid the battles in here if you can. Return to the room containing Kumool the moogle, slide down the pole and into the area below. Amarant is in here, attempt to move down to the bottom of this area, Zidane finds Amarant. Return to the entrance of Ipsen's Castle, Zidane will go over the objects that were found within the castle. Don't forget to switch back to your powerful items when you have the chance!