It is a common saying of the history teachers of Johto that Donphan are the reason why castles are built on hilltops. This is an exaggeration, for height confers enough advantages that castles are built on high ground even in lands far away from any Donphan. However, castles on flat plains are often toppled by these pokemon and there are few more terrifying sights in warfare than an avalanche of Donphan rolling downhill.
Other large, round pokemon, such as Electrode and Graveler, are often used in the same way as Donphan; heavy balls which plummet downhill with enough force to knock over anything from trees to attacking armies. But Donphan have two major advantages over these these other pokemon: its tusks, low to the ground, make its Rollout a wider attack in area with the potential to stab as well as send flying. It can also cease its rolling by extending its long, armored trunk, either to protect itself or to trip up its enemies, unlike rounder pokemon who are easily blocked by tower shields.
There was one reliable way to defeat these Donphan before the invention of cannon. I do not refer to water or grass attacks, which seldom have enough momentum to alter the course of a rolling Donphan, and a fainted Donphan which continues moving is nearly as dangerous as a conscious one! They do lose momentum going uphill, and if the incline is too steep the Donphan will simply be sent flying over the battlefield. Therefore, in the age of castles and Donphan, earthworks were probably the most important part of warfare, for altering the ground was the best defense against Rollout. (Flying pokemon were also used, but could be shot down too easily by Stone Edge.)
And few pokemon dig better earthworks than other Donphan.