Almanzor crosses the Duero and invades Castile near Madinat Selim, where he sights an army under Sancho García and the "Galician kings", consisting of troops from as far as Pamplona and Astorga (that count Sancho leads the group is thought to reflect the decline of García).
Almanzor passes by the fortresses of Osma, San Esteban de Gormaz, and Clunia, which had been in Muslim hands for several years at that point.
Just north of Clunia, he is surprised to find a large Christian army.
Sancho, who had been elected leader of the entire army by the assembled troops, is encamped at the rock of Yarbayra (Peña de Cervera), a central location, well situated for supplies, and inaccessible due to its geography.
The implication of these events is that Sancho's planning and communications had been superior to those of Almanzor.
The roads from Clunia, the northernmost Muslims garrison, lead to Tordómar, Lara, and Salas de los Infantes through the narrow pass of the Yecla, which passes through the Peña de Cervera before widening into the basin of the river Arlanza.
Almanzor is aware of his disadvantaged position—the Christians have a stronger camp and better sightlines, with a large open field before them—but his viziers cannot agree on how to proceed.
Then, without planning and without strategy, the Christian host descends on the unsuspecting Muslims and the battle soon evolves into a general mêlée.
Almanzor's right and left columns are attacked simultaneously and soon fall back, which inspires the Christians to press their advantage even more boldly.
The majority of the Muslim rear, disoriented by the unorganized attack and disheartened by the result, flee.
According to the eyewitness Jalaf ibn Husayn, Almanzor, with his escort, observed the action from atop a short hill near the field.
While he was debating whether to throw his retinue into the fight the right wing broke, which furthered the confusion.
One of Almanzor's secretaries, Abd al-Malik ibn Idris al-Yaziri, said to Said Ibn Yusuf, as reported by Ibn al-Qalina, "Come to say goodbye, oh martyr, because surely today you must die."
By the end of the day Said Ibn Yusuf was dead.
Jalaf also related how, when Almanzor asked him how many of his own retinue remained, Jalaf counted them off for him one by one and arrived at twenty.
Almanzor eventually dismounted and returned to his couch in order to strengthen the confidence of those around him.
It was there that Jalaf says he hit upon the idea of transferring the camp from the shallow depression in which he had first pitched it to the hill from which he had been observing the battle.
The sight of the large tent of Almanzor now propped on the hill demoralized the Christians and inspired the Muslims.
In their ten-mile flight, many Christians are captured and many more killed, while more than seven hundred Muslims die this day.
The Christian camp is captured and pillaged.
Ibn al-Khatib notes that the most outstanding fighter on the Muslim side was Abd al-Malik, Almanzor's son, stressing that this was "by unanimous opinion" without any favoritism and that he excelled even the famous Berber cavalry.
Among these, Kayaddayr al-Dammari al-Abra (the Leper), a prince of the north African tribe of the Banu Dammari, is most famous.
At Cervera he reputedly decapitated one of the counts of the Banu Gumis and carried his head about with him.
Ibn al-Khatib also praises Almanzor's second son by another woman, Abd al-Rahman, who follows his half-brother into battle.
Both brothers watch the battle with their father until he finally permits them, tearfully, to join it.
Though the battle is a Muslim victory, the Christians do succeed in preventing further inroads by Almanzor in Castile.
