The windmill is taking a long time and a huge amount of work to complete, but the animals are happy to do it. As he describes their feelings, Orwell uses a simile to underscore the intensity of their ceaseless, painful work:
All that year the animals worked like slaves. But they were happy in their work: they grudged no effort or sacrifice, well aware that everything that they did was for the benefit of themselves and those of their kind who would come after them.
The animals believe their relentless work benefits their collective future, but by this point the reader already knows—or at least strongly suspects—that the pigs do not have the other animals’ best interests at heart. Napoleon and his advisors are exploiting the animals’ trust in them for their own gain, not for the communal benefit of “those of their kind.” This conflict is where the dramatic irony lies: the animals’ sincere belief in their work's purpose is undercut by the reality that they are being used. They are “happy in their work” but have no idea what is actually happening behind the scenes.
The simile "worked like slaves" also contrasts the animals’ “effort” and “sacrifice” with their hopeful belief in the revolutionary promises of Animal Farm. Here, Orwell highlights the disparity between their perception of their situation and the reality of their exploitation. The animals are willing to push themselves because they believe it’ll be worth it. By likening their conditions to the work of enslaved people, though, the author underlines the fact that they are being exploited. This simile emphasizes that, like enslaved people, the animals actually have no agency in deciding their future.
In this passage from the middle of the novel, Orwell uses a simile to show the effect of natural and man-made “disasters” on the farm and the flag the animals have proudly hoisted. After a night of terrible weather, the following occurs:
In the morning the animals came out of their stalls to find that the flagstaff had been blown down and an elm tree at the foot of the orchard had been plucked up like a radish.
The simile "plucked up like a radish" makes the unexpected and effortless destruction of the elm tree feel real for the reader. Elm trees are large, strong trees with particularly deep roots, but the simile implies quite the opposite. Radishes, small and easily harvested, are very fragile and trivial in comparison to huge plants like Animal Farm’s elms. The fact that the tree has been plucked “like a radish” robs it of all of the usual robustness associated with trees. Seeing this makes the animals feel the sudden vulnerability of the farm itself, suggesting that the stability they believed they’d finally won was actually as easy to uproot as a radish.
While it’s not a direct metaphor, the toppling of the flagstaff here is also important. The fact that it’s “blown down” just as easily as the tree was “plucked up” points to the delicacy of the system the animals have built. The flag symbolizes their newfound autonomy. When it topples, it suggests that autonomy is in real peril.