Robert of Gloucester, the natural son of Henry, entertained the strongest feelings of hostility to Stephen. He appears, however, to have directed his efforts against the usurper rather in support of the claims of his sister Matilda, than of any pretensions of his own. On the elevation of Stephen to the throne, Robert found it necessary to take the oath of allegiance, since a refusal to do so would have resulted in the loss of his estates in England, and of that power which he proposed to use in his sister's behalf. He therefore offered to do homage on condition that the king fulfilled all his promises, and never invaded any of the rights of Robert. Thus a pretext was afforded for revolt at any moment, and the Earl of Gloucester, who was a man of considerable abilities and military reputation, occupied himself in promoting a spirit of disaffection among the nobles. The right which the English barons had obtained of erecting fortified castles was exercised to the utmost. Strong fortresses rapidly arose in all parts of the kingdom, and were garrisoned with licentious soldiery, native and foreign. In proportion as the privileges of the nobles were extended, the condition of the people became once more one of oppression and misery. Petty wars broke out among the rival barons, who made incursions into each others' territories, and practised unbounded rapine on the towns and villages. Some of the more powerful chiefs declared that the promises made to them by Stephen on his accession had not been fulfilled; and they seized various parts of the royal estates, which they asserted were their due. Among these was Hugh Bigod, whose act of perjury had secured the coronation of Stephen, and who now revolted openly against the king, and took possession of Norwich Castle. The insurgents had not yet learned to act in concert, and Stephen soon recovered the estates which had been seized. The spirit of sedition, however, was not repressed; new disturbances were continually taking place, and the country remained in a state of anarchy. In the year 1137, Robert, Earl of Gloucester, having organised an extensive confederacy, quitted his estates, and having crossed the Channel, sent to the king a formal letter of defiance. Other great barons also, on the ground that the promises made to them had not been fulfilled, renounced their homage, and retired to their strongholds. A desultory warfare took place between the king and his disaffected nobles.