A Tribe is a group of players who are allied.
Any player with a level 5 Town Center can form a level 1 tribe.
Tribes are limited in size by their level. Each level upgrade is paid for with contributions made at Trading Posts by tribe members. As tribe slots are restricted to five players per level, players are generally required to be active and of a certain level to join a tribe, although this is not an obligation.
Contributed resources can only be used to upgrade the tribe or repair Stronghold gates.
Tribes can coordinate attacks and defenses using Assignments. It is fairly common for major tribes to have wars involving unit counts in the thousands.
The ratio between resources is 10 Gold : 50 Wood : 25 Crop : 1 Iron.
The iron required per tribe member starts at 42, and is set with the formula $ 2L^2+40 $, where L is the current level. For instance, to go from level 3 to level 4, $ 2L^2+40 = 2*3^2+40 = 2*9+40 = 18+40 = 58 $. Multiply by the number of members in a level 3 tribe to get the total requirement. $ 5L*58 = 5*3*58 = 15*58 = 870 $.
Alternatively, you can use $ T(2T-4)+42 $, where T is the target level.