One of the rebel Barons excommunicated for backing the Magna Carta   [1] .

"It is impossible to follow the spread of the rebellion in detail. ... the King directed his men to seize the lands of his enemies... by now John was losing touch with his northern castellans... This gap in evidence cannot be filled... The one detailed list, that which Roger of Wendover gave of the men who met at Stamford at Easter, is thoroughly unreliable and was largely compiled from the later letters excommunicating the rebels which Wendover included in his chronicle...

... It reflects the papal letters of excommunication... More significantly, the names of Robert de Vere, Fulk fitz Warin, William Malet, and William de Montagu appear in the same contiguous order in both Wendover's list and the papal letters of excommunication." (Holt)

The Magna Carta   [1]   [2]  [3]  [4]  [5]  [6]  [7]   [8], is considered a pivotal historical development:

"... Now a civil war was being fought for a cause, a programme, not for one individual or even several, but for a document, a simple piece of parchment. The rebellion which King John faced was thus quite novel. It was the first of a long line which lead through the Provisions of 1258-9 and the Ordinances of 1311 down to the Grand Remonstrance of 1641. Of all these the Magna Carta was the ancestor and was so recognized by its progeny.

As a new departure it was fumbling, half-blind. Its provisions often constituted little more than crude propaganda... or administrative tinkering at once naive and impractical... Those who fought for it were as hesitant as their programme was ill-formed. Scarcely one maintained his course undiverted, without considering surrender, from the beginning of the crisis in 1212 to the end of the civil war in 1217. They looked for precedents for what they were doing... Their experience provided them with one great example of a war fought for a cause, and that was the Crusade. ... they described themselves as the Army of God and Holy Church...

The men who were responsible for the Great Charter of 1215 asserted one great principle. In their view the realm was more than a geographic or administrative unit. It was a community. As such, it was capable of possessing rights and liberties. Magna Carta was indeed a statement of these rights and liberties, which could be asserted against any member of the community, even and especially the King. The durability of the Magna Carta is to be explained by the general utility of this central concept. ... the constitutional history of England has been ... "a commentary on this charter"...

For this many different men were in some degree responsible. ... But the fight for the Charter was conducted by the rebel barons and none other." (Holt).

A castellan is a noble formally charged with maintaining and defending a castle.

Sources:
The Northerners, Holt.

http://www.montaguemillennium.com/familyresearch/h_1216_william.htm