This fragment explains how they could have done it:

The most common theory of how prehistoric people moved megaliths has them creating a track of logs which the large stones were rolled along.[49] Another megalith transport theory involves the use of a type of sleigh running on a track greased with animal fat.[49] Such an experiment with a sleigh carrying a 40-ton slab of stone was successfully conducted near Stonehenge in 1995. A team of more than 100 workers managed to push and pull the slab along the 18-mile (29 km) journey from the Marlborough Downs.[49]

My point was that it’s not difficult with modern machines at all. But it can also be done with the methods described above. Especially if you work on it for 1500 years.