Although I don’t know that J.R.R. Tolkien read Notker the Stammerer’s Deeds of Charles, I know that he was influenced by medieval literature. In Notker’s second book, there is a passage about the Charlemagne’s assault on the rebellious Lombard lord Desiderius that seemed like something one would read in Lord of the Rings:
Now it happened that some years before one of the first nobles, called Otker, had incurred the wrath of the most terrible emperor, and had fled for refuge to Desiderius. When the near approach of the dreaded Charles was known, these two went up into a very high tower, from which they could see anyone approaching at a very great distance. When therefore the baggage-waggons appeared, which moved more swiftly than those used by Darius or Julius, Desiderius said to Otker: “Is Charles in that vast army?” And Otker answered: “Not yet.” Then when he saw the vast force of the nations gathered together from all parts of his empire, he said with confidence to Otker: “Surely Charles moves in pride among those forces.” But Otker answered: “Not yet, not yet.” Then Desiderius fell into great alarm and said, “What shall we do if a yet great [sic] force comes with him?” And Otker said, “You will see what he is like when he comes. What will happen to us I cannot say.” And, behold, while they were thus talking, there came in sight Charles’s personal attendants, who never rested from their labours; and Desiderius saw them and cried in amazement, “There is Charles.” And Otker answered: “Not yet, not yet.” Then they saw the bishops and the abbots and the clerks of his chapel with their attendants. When he saw them he hated the light and longed for death, and sobbed and stammered, “Let us of down to hide ourselves in the earth from the face of an enemy so terrible.” And Otker answered trembling, of once, in happier days, he had had through and constant knowledge of the policy and preparations of the unconquerable Charles: “When you see an iron harvest bristling in the fields; and the Po and the Ticino pouring against the walls of the city like the waves of the sea, gleaming black with glint of iron, then know that Charles is at hand.” Hardly were these words finished when there came from the west a black cloud, which turned the bright day to horrid gloom. But as the emperor drew nearer the gleam of the arms turned the darkness into day, a day darker than any night to that beleaguered garrison. Then could be seen the iron Charles, helmeted with an iron helmet, his hands clad in iron gauntlets, his iron breast and broad shoulders protected with an iron breastplate: an iron spear was raised on high in his left hand; his right always rested on his unconquered iron falchion. The thighs, which with most men are uncovered that they may the more easily ride on horseback, were in his case clad with plates of iron: I need make no special mention of his greaves, for the greaves of all the army were of iron. His shield was all of iron: his charger was iron coloured and iron-hearted. All who went before him, all who marched by his side, all who followed after him and the whole equipment o the army imitated him as closely as possible. The fields and open places were filled with iron; the rays of the sun were thrown back by the gleam of iron; a people harder than iron paid universal honour to the hardness of iron. The horror of the dungeon seemed less than the bright gleam of iron. “Oh the iron! Woe for the iron!” was the confused cry that rose from the citizens. The strong walls shook at the sight of the iron; the resolution of young and old fell before the iron. Now when the truthful Otker saw in one swift glance all this which I, with stammering tongue and the voice of a child, have been clumsily explaining with rambling words, he said to Desiderius: “There is the Charles that you so much desired to see”: and when he had said this he fell to the ground half dead.
From Book II, Chapter 17. Passage quoted from the Internet Medieval Sourcebook (with page numbers in brackets taken out).
There are also echoes of 1 Kings 19. I don’t know if it was intentional but it really jumped out at me:
There he came to a cave and lodged in it. And behold, the word of the LORD came to him, and he said to him, “What are you doing here, Elijah?” He said, “I have been very jealous for the LORD, the God of hosts. For the people of Israel have forsaken your covenant, thrown down your altars, and killed your prophets with the sword, and I, even I only, am left, and they seek my life, to take it away.” And he said, “Go out and stand on the mount before the LORD.” And behold, the LORD passed by, and a great and strong wind tore the mountains and broke in pieces the rocks before the LORD, but the LORD was not in the wind. And after the wind an earthquake, but the LORD was not in the earthquake. And after the earthquake a fire, but the LORD was not in the fire. And after the fire the sound of a low whisper. And when Elijah heard it, he wrapped his face in his cloak and went out and stood at the entrance of the cave. And behold, there came a voice to him and said, “What are you doing here, Elijah?” He said, “I have been very jealous for the LORD, the God of hosts. For the people of Israel have forsaken your covenant, thrown down your altars, and killed your prophets with the sword, and I, even I only, am left, and they seek my life, to take it away.” And the LORD said to him, “Go, return on your way to the wilderness of Damascus. And when you arrive, you shall anoint Hazael to be king over Syria. And Jehu the son of Nimshi you shall anoint to be king over Israel, and Elisha the son of Shaphat of Abel-meholah you shall anoint to be prophet in your place. And the one who escapes from the sword of Hazael shall Jehu put to death, and the one who escapes from the sword of Jehu shall Elisha put to death. Yet I will leave seven thousand in Israel, all the knees that have not bowed to Baal, and every mouth that has not kissed him.”
(1 Kings 19:9-18 ESV)