This expensive, sweeping, surreal saga was an international production but most of the heavy lifting was done by the Soviets, who lent their land, 17,000 soldiers, their director and many millions of dollars to re-creating the battle.  From Wikipedia:

To recreate the battlefield authentically, the Soviets bulldozed away two hills, laid five miles of roads, transplanted 5,000 trees, sowed fields of rye, barley and wildflowers and reconstructed four historic buildings. To create the mud, more than six miles of underground irrigation piping was specially laid. Most of the battle scenes were filmed using five Panavision cameras simultaneously – from ground level, from 100-foot towers, from a helicopter, and from an overhead railway built right across the location.

Actual filming was accomplished over 28 weeks, which included 16 days of delay (principally due to bad weather). Many of the battle scenes were filmed in the summer of 1969 in often sweltering heat. In addition to the battlefield in Ukraine, filming also took place on location in Royal Palace of Caserta, Italy, while interior scenes were filmed on the large De Laurentiis Studios lot in Rome. The battle sequences of the film include about 15,000 Soviet foot soldiers and 2,000 cavalrymen as extras and 50 circus stunt riders were used to perform the dangerous horse falls. It has been joked that Sergei Bondarchuk was in command of the seventh-largest army in the world.  Months before the cameras started filming, the 17,000 soldiers began training to learn 1815 drill and battle formations, as well as the use of sabres, bayonets and handling cannons. A selected 2,000 additional men were also taught to load and fire muskets. This army lived in a large encampment next to the battlefield. Each day after breakfast, they marched to a large wardrobe building, donned their French, British or Prussian uniforms and fifteen minutes later were in position. The soldiers were commanded by officers who took orders from director Sergei Bondarchuk via walkie-talkie. To assist in the direction of this huge, multi-national undertaking, the Soviet-Ukrainian director had four interpreters permanently at his side: one each for English, Italian, French and Serbo-Croatian.

The expanse of the endeavor is breathtaking and the efforts of the Soviets are plainly evident in the cinematography:

The script is, well . . .  dated.  The players intone with great import, and before most lines, they damn near lean into the frame.   As Wellington, Christopher Plummer is so effete and aristocratic, he approaches the Monty Pythonesque.  I implore you, go to 4:03 of the above scene for my brief in support.  Yet, somehow, he works.  Rod Steiger’s Napoleon is a raving consumer of all things Lee Strasberg, yet he too seems to work.  Indeed, one of the charming qualities of the script is when, in the middle of the goddawful melee, the soundtrack goes silent and we hear each man’s thoughts in voice-over.  “Who is this man, who fights on his ass,” Napoleon muses as he watches Wellington dig in.

Full disclosure:  this move was a staple on the 4 o’clock move when I was growing up and along with Zulu and Where Eagles Dare and countless other war pictures, informed my young sensibilities in the areas of hyper-masculinity, glory, bravery under fire and all the rest of it.  White collar life is empty of such things, so my emotional nostalgia may be at play here.

Still, it is really a wondrous picture to watch.