In fact the resources system work. If you mouse over the time needed for a building you will see what it needs.
For instance a house need 5 workforce. You can't go lower that number of turn (at the moment. Maybe a letr build will allow us to decrease that number)
But ! Some buildings need workforce AND lumber like the watch tower. If you don't have any lumber mill it will take 36 turns to build a wtach tower : 15 workforce + 16 lumber. If you don't have the resource it is replaced by workforce. If you have the resource, for instance 4 lumber, it will need 15 workforce + 12 lumber (4 lumber already there) = 27 turns.
Try it : buid a watch tower without any lumbermill constructed, then build acity near forest, build two lumbermill, then a watch tower. If you click on the watchtower before the lumber mills are built, then the game will calculate the long number of turns. But as soon as the lumber mills are built, the turns needed for the watchtower will decrease.