Final Fantasy IV/Tower of Babil

Underground Passage
Leave the throne room, then in the next room head to the staircase on the right. In that room you can buy some upgraded weapons and armor if you would like. After you do that, head to the stair case in the upperleft of the room. In this room go left and in the middle of the room you will see a Gnome in front of a secret room. Speak to him and he will open up the Underground Passage for you. Continue through the passage and you will eventually find yourself outside.

Find Tower of Babil
Once outside, go to the left of the door and head along the coast as far as possible. Now head straight up (ignoring all splits in the road and continuing north) until you can see some tanks. Head to the right side of the tanks. The tanks will attack the tower, allowing you to get through.

Ice Spear and Sword
Inside the tower head straight through into the door ahead of you. In the next room follow the path until you get to the first door you come to. Go inside, the chest contains monsters, who drop a Blizzard Spear when you defeat them. This will be important for fighting Rubicant. Continue on to the door above you to the right. This will be a chest with monsters who drop the Ice sword. Continue on to the only other door in the room.

Next Level
In the next room you will need to go to the right and take the first path north available. Continue all the way around until you reach the next door.