Besides his multitudinous correspondence with the public bodies, whose labors he habitually directed; with the various estates of the provinces, which he was gradually moulding into an organised and general resistance to the Spanish power; with public envoys and with secret agents to foreign cabinets, all of whom received their instructions from him alone; with individuals of eminence and influence, whom he was eloquently urging to abandon their hostile position to their fatherland; and to assist him in the great work which he was doing; besides these numerous avocations, he was actively and anxiously engaged during the spring of 1576, with the attempt to relieve the city of Zierickzee.
That important place, the capital of Schouwen, and the key to half Zealand, had remained closely invested since the memorable expedition to Duiveland.

The Prince had passed much of his time in the neighbourhood, during the month of May, in order to attend personally to the contemplated relief, and to correspond daily with the beleaguered garrison.

At last, on the 25th of May, a vigorous effort was made to throw in succor by sea.

The brave Admiral Boisot, hero of the memorable relief of Leyden, had charge of the expedition.

Mondragon had surrounded the shallow harbor with hulks and chains, and with a loose submerged dyke of piles and rubbish.