Ah, so you're having trouble entering Dragontooth dungeon, huh? Well, your problems are over. First, here's how you were supposed to find out.

As you probably know, the messages on the statues are the key, since they speak of the Werewolf Wars and you need the number of the castle guard that were still alive after these Wars. The statue at the entrance tells you how many there were to start with: "20 legions of the castle's guard drove the werewolves from Serpent Wood." North from that statue is another statue whose inscription reads: "10 legions chased the werewolves across the Great Sea to the Frozen Isles." So apparently, 10 legions didn't survive, and there's 10 left. The next statue is close to Pathos. The inscription reads: "A great battle began as the werewolves turned to make their final stand." That doesn't tell you much. Another statue in the room with the Dark Pegasi lets you know that "5 legions fell before the werewolves were finally driven to the mountains." So now there's 5 legions left. However, that still doesn't tell you anything about how many soldiers form a legion.

To find out that last bit of info, you must teleport to a secret room in the southeast corner of the castle. There's another statue there, and the inscription says: "Erected in the honor of Silver Fang, hero of the rout of the werewolves and leader of 4 legions. (16,000 of the castle guard)" So that means 4000 guards make up one legion. Doing the math, you now know that the number of castle guards remaining after the Werewolf Wars, and thus the passcode to the dungeon, is 5 x 4000 = 20000.