A force of 24,000 infantry and 4000 cavalry were gathered together for the enterprise.

It was composed of many nationalities, like all the armies commanded by Maurice and Frederick Henry, but was admirably disciplined and devoted to its commander.

Four English, three Scottish and four French regiments, all choice troops, raised by permission of their sovereigns for the service of the States, formed the backbone of the force.

On April 30 the town was invested.
Hertogenbosch, or Bois-le-duc, was strongly fortified, and so surrounded by marshy ground, intersected by a number of small streams, that the only way of approach for a besieging force was a single causeway defended by the forts of St Isabella and St Anthony.

The garrison consisted of 8000 men, and the governor, Grobendonc, was an experienced and resolute soldier.
The stadholder began by surrounding the town with a double line of circumvallation.