During the whole of November the siege of Kinsale was pressed with the utmost vigour by Mountjoy. The place mounted but three or four effective guns, while 20 great pieces of ordnance were continually playing on the walls. On the 1st of December a breach was found practicable, and an assault made by a party of 2,000 English was bravely repulsed by the Spaniards. The English fleet, ordered round to Castlehaven on the 3rd, were becalmed, and suffered some damage from a battery, manned by Spanish gunners, on the shore. The lines were advanced closer towards the town, and the bombardment became more effective. But the English ranks were considerably thinned by disease and desertion, so that on the last day of December, when the united Irish force took up their position at Belgoley, a mile to the north of their lines, the Lord Deputy's effective force did not, it is thought, exceed 10,000 men. The Catholic army has generally been estimated at 6,000 native foot and 500 horse; to these are to be added 300 Spaniards, under Don Alphonso Ocampo, who joined O'Donnell at Castlehaven.
The prospect for the besiegers was becoming exceedingly critical, but the Spaniards in Kinsale were far from being satisfied with their position. They had been fully three months within walls, in a region wholly unknown to them before their allies appeared. They neither understood nor made allowance for the immense difficulties of a winter campaign in a country trenched with innumerable swollen streams, thick with woods, which, at that season, gave no shelter, and where camping out at nights was enough to chill the hottest blood. They only felt their own inconveniences: they were cut off from escape by sea by a powerful English fleet, and Carew was already practising indirectly on their commander his "wit and cunning," in the fabrication of rumours, and the forging of letters. Don Juan wrote urgent appeals to the northern chiefs to attack the English lines without another day's delay, and a council of war, the third day after their arrival at Belgoley, decided that the attack should be made on the morrow. This decision was come to on the motion of O'Donnell, contrary to the judgment of the more circumspect and far-seeing O'Neil. Overruled, the latter acquiesced in the decision, and cheerfully prepared to discharge his duty.