To further expand on the two relevant (and correct) answers, the levels must be unlocked every week with goods from the guild Treasury. If the guild Treasury does not contain enough of the needed goods, the game will ask for donations when you go to the unlock screen. The goods needed are based on the age/era of all guild members Iron Age or above, active or inactive. For each Iron Age guild member, a certain number of Iron Age goods are needed to unlock each level. For every guild member in EMA or above, it requires previous era goods for level 2 and current era goods for levels 3 and 4. Levels can only be unlocked for the entire guild, not for individual players, meaning that your guild Treasury must contain all needed goods from every era necessary in order to unlock a level.