This first area consists largely of a single tunnel infested with Hoplites. There is a side tunnel off to the left near the end, which leads to a Juggernaut guarding a Platinum Bangle and the Cie'th stone for Mission 29. The Juggernaut is likely too difficult at this point unless you spent some time leveling and doing Missions in The Archylte Steppe. However, once defeated, it will not reappear if you leave the area and return. At the end of the main path, you'll find a Boxed Phalanx paired with Hoplites, which can be a very difficult battle if you haven't spent much time leveling in Chapter 11. If you can't manage a victory, try summoning an Eidolon.