Duke Leopold III. of Austria, after he unsuccesfully tried to establish a cheap peace, decided to assemble his forces in order to save possessions and honor of his house. With the help from Tyrol and Italy his army was considerably stronger than that of his uncle Leopold I. at Morgarten. His strenght is estimated to be between 3000 and 4000 men.
But the Confederation's army was also probably twice as strong as at Morgarten and numbered somewhere between 6000 and 8000 men, since it was made not only of men from the four forest cantons, but also other Schwyz cantons (Lucerne, Zurich).
Leopold wisely decided not to attack principal places and turned to the small town of Sempach, some 9 to 10 miles north of Lucerne.
He assembled his army at Sursee, about 5 miles down from Sempach, surrounded Sempach and on the same day started to march towards the expected relief army.
He did not take the direct route to Lucerne, but rather turned east. He must have known that an enemy army is approaching from there.
The Confederation army has presumabely assembled at the bridge over the Reuss at Gislikon. It marched from there, hoping that it would catch Leopold still at Sempach where he could be pressed against the lake.
Towards the noon the points of the two armies made contact a short half-hour above Sempach, close to the village of Hildisrieden. The battlefield is definitely proven to be by the old battle chapel.
As the knights of the Leopold's army came up, they dismounted and sought to storm the high ground. Their marksmen took the Swiss under heavy fire.
Leopold, probably thinking that all the Swiss army is already in front of him, participated in the battle before his rear units moved up from the march column. The knights drove with such force into the enemy that the Lucerne banner had fallen. But it was only the confederate's advance guard that they were fighting.
The main body of the Confederation army, probably somewhat delayed, finally completed its deployment from the march column, formed up, and suddenly attacked the knights from the flank with a great stout and many stones thrown in front of it.
The attack was so poverful that the knights fighting on foot were immediately overrun.
The soldiers, who were holding knights' horses, took flight and that part of the Habsburg army that was still on horseback was carried away by the fugitives.
Duke Leopold and with him a large number of nobles and knights were slain.
(after Delbrück, Medieval Warfare, Univ. of Nebraska Press 1990)