In 1628 one of the many schemes undertaken met with spectacular success when Admiral Piet Hein of the Dutch West India Company captured the Spanish treasure fleet.

The vastly improved financial situation of the Republic allowed for a major counter-strike and Stadtholder Frederick Henry decided to break the Habsburg morale by conquering their main stronghold in the Netherlands.

He diverted the two main streams feeding the swamps (the Dommel and the Aa) around the city by means of a double forty kilometre dike, in the form of a giant square, completely enclosing the fortress.

They sent a large relief army under command of Frederick Henry's Catholic cousin Hendrik van den Bergh, reaching the city in July.

Van den Bergh quickly found out that his cousin's circumvallation, successfully tested at the siege of Grol in 1627, was too strong to breach.

Bishop Michael Ophovius tried to plead with Frederick Henry for religious tolerance, but though the stadtholder himself would gladly have granted religious freedom to the Catholics, as he tried to gain popularity in the Spanish Netherlands in the hope support for the rebellion would grow, due to vehement Calvinist resistance it had already been decided to be as strict in this respect as in the rest of the Republic; only nunneries could remain until the last nun of those present in 1629 had died.

[3] As a result, their position in the North would soon crumble and Frederick Henry, encouraged by the success, would start a series of other sieges, assisted by his cousin Van den Bergh, who changed sides after being accused of treason.