Source E tells us a lot about why the attack failed. Where the source says “There a halt was made and it was found that there was a considerable pass which it was judged should be occupied. This the Count van Hoorn did and remained there with four of five hundred.” This means that four or five hundred men were left to guard a pass and so were not available for the attack on the fort. At the time it was not thought that these men would be needed for a victory as the Dutch had underestimated the number of men and big guns at the fort.
On reaching the fort Colonel Dolman discovered “Firstly, the fort appeared to be fortified with four good bastions or bulwarks, partly with earth and the rest with masonry with a good ditch” and “Secondly, it was fortified with a good covered way and counterscarp, leaving those standing behind it in safety. The covered way was filled with men, who were well disciplined and alert and were faithfully keeping a lookout” for the Dutch.