The construction of some special buildings like the Ancient Portal, Statue of Fertility or Water Mill is rather time and resource-consuming. Every such building goes through several stages, and each stage consists of five levels.To finish the building and unlock its unique features, you need not only to complete the initial five levels, but also to set up all the stages, whose total number can be up to ten. Moreover, you can receive additional quests between different levels or stages that you have to complete in order to proceed with the construction.
Be patient and persistent, and you're sure to complete the desired building!