These were not prehistoric people, they were fully evolved creative innovators like we are today.  They could have lifted blocks one by one, but I seriously doubt they didn't also think to lift any tools, machines, animals or water to help the job get done before the emporer dies. 🤷‍♂️

For kicks, ask AI to generate some possible solutions besides pure slave labor that could conceivably have taken place, given Idunno, 50 or 100 years to beat the deadline of the kings death?