Prerequisites: Main Building Level 1

The embassy is the place for diplomatic relations. At level 1 you can join an alliance, while with a level 3 embassy you may even found one yourself.

There is no longer an embassy level requirement for alliances.  All alliances can have up to 60 players irrespective of embassy level.

The level dependent attributes (such as construction costs, generated culture points and maximum alliance members) can be found here.
For the full table of construction times clickthis link for server with 1x speed, this link for server with 2x speed and this link for server with 3x speed.