consider the symbol of the windmill

The windmill symbolizes Napoleon's manipulation of the other animals for his own gain. I think that Snowball, and inventor of idea, really meant the windmill to help the animals by drastically cutting their workload. After being chased off Napoleon, who formerly opposed it, claims it his brainchild. The windmill hence becomes a symbol of tyranny over the animals. They are made to work like slaves, Boxer in particular, for little to no profit for them. Napoleon simply exercised his power and took the profits for himself.