The two instances are similar in that they both scale to a player's level between 40 and 80, and they're both accessed via the Gateway to Khitai zone that serves as the entry point into AoC's Rise of the Godslayer content. That's about where the similarities end, though, as the dungeons are quite different in terms of visual style and challenge level.
covered a bit of the lore behind the Forgotten City instance, so I won't spend too much time on that here. Suffice it to say that a crazy Lemurian admiral took it upon himself to construct a walled city, the better to hide the secret of eternal life discovered there and keep it to himself (and his followers). As lore goes, it's an interesting setup, but knowing about it is, of course, completely optional since the goal of the instance can be summed up as follow-the-path-and-kill-what-moves.
Accessing the instance is more straight-forward than it was for the Breach. If you're in the northern portion of the Gateway zone and you've got your draw distance set basically anywhere above the minimum, you'll see the Forgotten City rising out of the foothills (and if that doesn't work, it's marked on your map).
Navigating to the entrance is pretty simple, as there are multiple paths and there's no where-do-I-stand-to-activate-the-instance confusion like there was with the Breach.
There are a few mobs on the various pathways outside the city, chiefly 34ish merciless wights and 33ish scorpions, but they're easy to pull in pairs and should be no trouble even for under-geared level 40 players.
The entrance is marked by a large and foreboding gate, as well as an NPC named Jiuling standing off to one side. He gives the Older Than Memory quest, which is basically a bit of grave-robbing that involves your character collecting a few of the relics scattered around the first portion of the instance.
One thing to note here is that you shouldn't be too eager to turn in said quest immediately after completing it, as zoning out to talk to Jiuling will net you some XP but it will also lock you out of the Forgotten City until the following day (and yes, I found this out the hard way, derp).
Inside, you'll find a path much like the one that weaves its way through the Breach instance. The atmosphere in the Forgotten City is unique, though, and dare I say more appealing. It's not raining, for one, though there's an eerie bit of ground fog that lends a supernatural chill to the place. Along the rocky pathway you'll encounter a bunch of sabretooths, scorpions, and fallen explorers, all of whom are non-elites and scaled to your level.
After you've collected your relics, you'll come upon the first miniboss guarding the entrance to the city proper. The architecture is pleasingly ancient, overgrown with vines and vegetation, and right off the set of a Wuxia martial arts film. The miniboss was something of a joke for my level 80 Demonologist, and though I haven't had a chance to play through the instance on my 40 Bear Shaman as of yet, I suspect he'd roll through it with almost as much ease.
Inside the archway you'll meet the first of the Lemurian admiral's guards, and the remainder of the instance is populated by ancient watchmen, magi, and oracle mobs, none of which will cause your character to break a sweat. They do look beautiful though, and I was somewhat disappointed to come away from the instance without a scrap or two of their armor.
As you step through the archway and out onto a terrace overlooking the whole of the Forgotten City, don't forget to click on the discarded journal page at your feet which starts The Price of Eternity quest. Completing it is a simple matter of finding the rest of the journal pages scattered around the instance, and if you're anything like me, you'll want to poke your virtual head into every nook and cranny due to the sheer grandeur of the place.
There's another miniboss in your immediate future as well as a room with six or seven mobs that might give you a bit of trouble until you figure out the environmental trick to dealing with them. After that, it's a matter of staying on the path and killing your way to the big kahuna at the instance's end.
Aiji the Unending isn't too intimidating, and he did in fact end -- rather quickly thanks to my Demo's DPS. Once you've disposed of his imminence and looted the chest that drops, you're basically done with the Forgotten City unless you want to traipse around and snap a few dozen screenshots.
Overall I had fun with my initial run-through of the Forgotten City. The challenge level is negligible and it seems like there are a few less mobs than the Breach instance (resulting in a bit less XP), but it was nonetheless an enjoyable romp that I'll gladly add to my daily Age of Conan rotation. Visually, Funcom has outdone itself yet again, and my only real complaint is that the instance is restricted to solo players and therefore inaccessible for the odd roleplay encounter.
In a nutshell, if you're looking for something to break up your mid-level AoC grind, I highly recommend doing both the Forgotten City and the Breach solo instances on a daily basis. Once you complete them a time or two, you'll be able to breeze through in 20 or 30 minutes, grab a couple of blue drops, get some decent XP, and enjoy a welcome change of scenery.