Great expansion for an already great game.
The Elder Sign base game is easily my favorite board/tabletop game that I have played. Unlike Unseen Forces, which added new investigators, adventure cards, Ancient Ones, and the Blessed/Cursed dice, this new expansion actually changes the "feel" of the base game. In fact, to play it properly, the instructions tell you to completely replace the base adventure cards and mythos deck. You can still play the base game, but some of the new items, investigators and Ancient Ones will mention rules and scenarios that only occur when playing in the city of Arkham, so you will have to disregard a few cards while playing the base game (since you are required to shuffle in the new cards into the base decks - the exceptions being the mythos and adventure card decks).
This expansion makes the game feel thematically much more like its big cousin Arkham Horror. Instead of taking place in the museum, the expansion opens the game up to the entire city of Arkham. All of the new adventure cards now have a face-up and a face-down side. One side shows the normal adventure cards we are accustomed to. On the back of these new cards are single locations from Arkham, such as Hibb's Roadhouse, or Arkham Asylum. When new adventure cards are added after winning an adventure, all new cards are added face-down, so you only see the location, and a color code, which hints at the adventure's difficulty. This is probably the biggest change to the base game, gameplay-wise. Now, you can no longer look at the 6 cards and plan which adventures you character is best suited for. You can still do this, but you essentially have to fail an adventure to have the next player see the card face-up. Also, the game starts with 3 of the 6 cards face-up. It makes more sense thematically this way, since it gives you that fear of the unknown, which is perfect for a game based on Lovecraft. Some face-down adventure cards have buffs and Midnight Effects that can be dealt with before turning over the card. The former becomes very important because the expansion removes the entrance cards that let you heal and buy items. Again, this helps with theme and immersion, since if you want to heal your sanity, you need that Arkham Asylum card to appear. You can no longer just go to the entrance and pay to heal up. (You can still heal one for free per turn.) You now have to explore the town to do these types of things in the game. This is the best part of the expansion, and is what makes the game feel that much more like Arkham Horror (but still much lighter).
The game adds 2 organizations you can join, the Order of the Silver Twilight, and the Sheldon Gang. You can only be in one group at any given time. These act as buffs in the game (similar to being blessed). Some adventure cards have tasks that show the icon of one of them. If you are a member, you automatically count that task as completed. Again, this adds to the theme, since the locations tend to match which groups are at which location. For example, the location "The Woods" has an adventure card titled "Moonshine By Moonlight." The Sheldon Gang is at this location, which makes thematic sense. In addition, some adventure card rewards are only available if you are a member.
The last major difference is the addition of gates. This is probably the most unnecessary addition, but it does not hurt the game. Some events or cards will cause Other World cards to open. With this expansion, opening an Other World card opens a gate (matching colored cardboard circles) on the Other World card that was just opened, and a randomly chosen regular adventure card (similar to how monsters are added). This essentially closes the regular location. You have to go through the Other World card and win the adventure to seal the gate, allowing you to enter the normal adventure card again, and preventing gates from opening again on that location. This was done to make it feel more like Arkham Horror, but thematically, I already felt like the Other World cards were gates themselves. I felt this was unnecessary since unlike Arkham Horror, closing gates is not the goal - finding Elder Signs is. But again, I do not think it hurts the game.
There are also new event cards (essentially a secondary mythos card, but without the additional Midnight Effect) that are played by certain mythos or adventure cards, and a new item card type called Skills. Skill cards have abilities that are similar to the investigator's abilities, but anyone can use them.
Overall, this is a very well done expansion. The only downside I see is that if you are teaching this game to new players, you will want to start with the base game, but adding this expansion litters the cards with things that do not apply to the base game. You can take all the expansion cards out, but that can be a pain. This applies to most games with expansions, though, so it is not really a big deal. It only bothers me a little since this was my go-to gateway game to get people into heavier games. I like to start out with Forbidden Island/Desert, Elder Sign, and if they are still hanging in there (with a few heavier games in between), Arkham Horror! This expansion gets my highest recommendation.
Edit: One last note on components. As usual, components for this game are fantastic, with one minor thing to note- There are little plastic stands meant to be used to prop up the cardboard gate discs vertically. I do not play with these since constant assembly/disassembly of these would eventually damage the cardboard discs. I put one together, and after pulling it apart, I could already see a dent in the cardboard. So now I just lay the gate discs flat on the board. Just a heads up if you do not want to cause eventual damage to them.
The search for elder signs leads you out of the museum and into the legendary and cursed city of Arkham in the Gates of Arkham expansion for Elder Sign. A new deck of Arkham Adventure cards replaces the core set deck and completely transforms the game, taking you to infamous locations such as Velma's Diner, the Curiositie Shoppe and the Uninvited Isle. Meanwhile, gates to Other Worlds are opening throughout the city, destabilizing time and space as new Ancient Ones threaten humanity. Join forces with eight additional investigators, using new skills and memberships too? nd the enduring advantages you need to survive Arkham's rough streets and prevent the Ancient Ones from awakening.