Siege warfare placed defenders at a distinct disadvantage right off:
Their enemies knew exactly where they were.
What their enemies did not know, however, was how many were the defenders, how well provisioned were they, and if there was some unseen weakness that could be exploited. Plus, there was a little nagging worry about the to-be-besieged having a field army somewhere in the area that could come to the rescue. Such had to be discovered before laying a siege.
The foolish would-be attacker tipped off defenders by sending in reconnaissance parties to look about in the day - or by marching in the entire army. Gathering information about an artillery fort prior to laying siege to it was usually done discretely, making use of spies and talkative neighbors. What needed to be discovered and/or confirmed was the size of the defending garrison, how many cannon and what size, what was the status of their supplies, including ammunition, and any weak features not visible from the outside. If scouts were sent in, they concealed themselves under the darkness of night. Samuel Marolois advised: "... it is necessarie, that one should get an exact draught, as well of the interiour, as of the exteriour part of the towne, or fortresse, which one resolves to approach unto, with all the markes, and observations, as namely, of hedges, wayes, hills, valleys, and the like, that ye maye know the better how to order and runne your approaches... The next thing is for you, to knowe the weakest part, and side of the towne, or place, where you may beginn your approaches."*
Once a fortification became the object of attack, then the attackers had to move swiftly to be successful.
*Samuel Marolois, The Art of Fortification, 1638, pgs 33 & 34