Note the little notch in the bottom of the moat illustrated here. A trench, dubbed a 'cuvette,' could be dug along the ditch bottom. After the ditch filled to become a moat, the water may still have been shallow enough to permit attackers to wade across, even in chest-high water. Unseen to them, though, was that underwater trench. Stepping into it, they would have stumbled or even gone under. Cuvettes were sometimes even dug for dry ditches as additional hindrances to retard attackers' rush to the ramparts.
To slow attackers another way, a fence of closely spaced posts or stakes was sometimes set all around the outside of the fortification. These could be posts in the moat, at the base of the ramparts or set horizontally near the top of the ramparts.
Deep into the siege, gaps may have been opened with artillery in fences like these. But one may surmise those gaps could have been to the defense's advantage by "encouraging" attackers in boats or on foot to go toward the openings, at which the besieged knew to concentrate their cannon and musket fire.
Posts along the ramparts' base were sometimes upgraded with solid, low walls, the fausse-braye, in particular around masonry forts. Providing an additional line of defense, musketeers hunkered down behind this lower position to pick off attackers approaching the edge of the moat or ditch.
A foul touch added to the moat was the presence of outhouses over the still, murky waters. At first sight one would consider this merely a convenience for waste disposal, but it added a psychological deterrence: announcement was made to any attacker that the water they wished to set foot in was sewage.