Rocksbourg's facade has crumbled. The "invasion" is here. Martial law is declared. Civilians are being herded into "safe zones," while elsewhere the various armed factions of the city fight to keep the undead at bay. After saving Garrett's life, the Keepers task him to run some errands in the locked-down part of Rocksbourg, one of them will send him into the domain of a mysterious sect...
The second mission in the Rocksbourg series is another city mission but it's very different from the first. Whereas the first is a single grimy urban maze with frantic gameplay at the start (before becoming a lengthy key scrounge), "Discovery" opens with a slow burn, with Garrett waking up in a small room in one of the safe zones and being told to take a walk to the inn for further instrictions. The streets are crowded with refugees and soldiers, the latter of which won't bother you if you don't make trouble. A lot of work was done to make this section convincing, depicting a cross section of the city's population in trouble: you'll find the usual beggars and drunks, right alongside more wealthy (or perhaps now, formerly wealthy) people who are at least for now, just as lost. There's even a sketch of improvised camp in a courtyard near the inn, for those who can't afford it, or perhaps there isn't room, because it's stuffed. Since you'll haveto backtrack to the safe zone at least two or three times, the level of eye candy and environmental storytelling is justified.
There are two locked-down zones you'll explore in the course of your mission. The first is sort of a warm-up as you track down a key to the second, consisting of a tall mechanist facility and a nicely vertical adjacent section of town. While not strenuous in terms of difficulty, the only real worry is that some of the guards in this zone are close enough to the safe zone to be overhead, so if you alert them you alert both. This actually happened to me the first time I played this mission and I found it more than doubled the difficulty of the map. I say that as more of an observation than a criticism: the gameplay that can emerges when you screw up is part of what makes Thief what it is, but it can play out in many interesting way depending on decisions made by the level designer.
The other section is bigger and more distant, connected to the safe zone by a creepy sewer, and has a more open layout similar to the the first Rocksbourg mission, albeit less initially confusing. There aren't actually any undead but guards are plentiful and here more challenging to evade. There are a few different objectives as well as hidden rooms to look for, but it's worth doing, as all DrK maps are dense with atmosphere, but you'll need to make your search thorough, which you probably already know if you made it through the sewer.
Yes, as in the previous mission, there's some difficult item hunts involved, this time not for keys but (mostly) hidden switches. You are blatantly given a clue for one of them, but it's arguable whether the others are in "required secret" territory. I suppose finally finding a key to an area that turns out to have no other obvious exit is clue enough that something's hidden inside, but it's not much to go one when the area is a certain size. It's not a mission-breaking issue by any means, but it still sticks out to me as an unfortunate and arbitrary brake on progression.
If you played the first Rocksbourg mission you'll remember the Silent Hill horror section near the end. There's one major surprise in this one that provides hints as to its nature, introduces a new faction to the Thief universe, and sets up the next mission, all at the same time.