the Embassy

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The Embassy is where you join an alliance in the beginning of the game, but after that, it's primary function it accepting and holding reinforcements from other alliance members. This serves as a means for people to protect you, especially when you're not online. I recommend always keeping your gates open, and your embassy open, so that alliance members can come in and help, even if you're not online. On a side note, do your fellow alliance members a favor and get your combat research done as soon as possible, as their troops will fight with your technology levels when they are inside your town.