I can try to help you get Excalibur (which I have obtained) but I have not reached the Zodiak Estucheon yet.

Okay, so you know how to get to XX, correct? From there you must go to the Sagitarrius gate stone (there are two paths - take either one... it loops back). Once you touch the stone, race back to XX (you are on a time limit) and head for Sagitarrius Gate II. Notice, there are two Sagitarrius Gates at platform XX. You want to touch Gate II before time runs out to open it. If, for some reason, Gate II will not open on the first try, open Gate I first. Then, repeat the process to open Gate II. You cannot have both Gates open at the same time, so one will close when the other is open.

Now, once you have Sagitarrius Gate II open, head through it to XII. Use the waystone to go to XIV. From there, two path are available. One leads up, one leads down. Go down. If you see the Leo Gate stone, you've gone the wrong way! Three platforms later, two paths are available. Take the path on the right. It leads to a dead end with a treasure, which is the Excalibur room.

Now, the treasure there is random, so you may not get Excalibur on the first try. Also, you must have at least one member of your party equipped with a diamond armlet to get it. If you fail to get it, go back at least three platforms to reset the treasure and try again.

It took me about half a dozen tries to get it, but it is worth the effort. Let me know if this helped or if you need more details?!