Charlemagne ended his reign as he began it: by fighting to defend his capital at Aix la Chapelle (now Aachen). First he fought the neighboring Saxons. In the end, he fought the Danes, who were part of the Scandinavian expansion that grew with the onset of a five-century warming trend (800 to 1300). By then he was an old man of 70. The Danes exhausted him. He was carried back on a litter and died in 814.
Much transpired between his first and final defense of the capital. Charlemagne built a stone palace with a chapel copied from one in Ravenna. (Chapels were places to keep Christian “relics.”) He built other palaces at strategic locations in the empire, such as Regensburg on the Danube at it most northern point. He started a canal between the Rhine and Danube to tie his empire together. He built a navy in the North Sea and in the Adriatic.
Charlemagne’s most lasting contributions were in education. He brought scholars to the palace school headed by Alcuin from York. Theodulf the Goth from Spain was the court theologian. In his writings he defended Filioque, the Son, which put Christ on the same level as the Father and Holy Spirit in the Trinity.
Charlemagne also ordered that parish schools actually teach and that cathedral schools be established to fill the gap between the parish and monastery. He ordered that the priesthood be reformed according to instructions of Pope Hadrian, who worked with Charlemagne for 20 years while he was Pope. With all of this, plus Carolingian script and musical notation, rudimentary foundations were laid in 800 that produced the renaissance of 1200, when many cathedral schools became universities.