Heights proper is a mountain range 1, 100 feet high, running nearly north and south, and ending in an almost perpendicular cliff opposite the mouth of the Shenandoah. About 1 mile from the Potomac, and at the highest point of the mountain, a double line of rifle -pits had been thrown across the crest of the ridge, extending well down on the west slope at this point being very steep for about 100 feet vertically. Between these rifle - pits and on the crest a structure of dry masonry known as the stone fort had been built, 100 by 40 feet, with two square bastions, the wall being from 4, 1/2 to 7 feet in thickness . Using the northern rifle - pit as a parapet, seven light guns, principally howitzers, had been placed in position, raking the crest of the mountain to the northward. This was the only preparation that had been made to resists an attack from that side . Near souther end of Maryland Heights, and 300 feet below the highest point, was a well -constructed battery mounting six 30 pounder Parrott guns and two 24 -pounder guns. This battery known as the 30- pounder battery, commanded perfectly the summit of Loudoun Heights opposite, as well as Bolivar Heights. On the western slope of Maryland Heights, about half way to the summit and near the river, was the naval battery, consisting of two 100-pounders and two 50 -pounder Dahlgren guns and two 24-pounders guns . This battery, also commanded Bolivar Heights and imperfectly the railroad bridge and the river front of Harper's Ferry. The timber on the crest of Maryland Heights had been cut from a point about 800 yards north of the stone fort to the 30 pounder battery, and on the west slope of the mountain from the stone fort south nearly Potomac . It will be observed that all the defenses, with the exception of the battery on the crest of Maryland Heights, had been made with a view of an attack from the south or across the Potomac, while the river itself at most seasons of the year afforded an almost impassable barrier, thus making it evident that the most feasible pint of attack was from the north. The problem, therefore, to be solved was to change the defenses so as to make them effective against an attack from that direction and at the same time not to weaken the defense from the south. The work of strengthening Fort Duncan and removing the guns from the battery south of it and mounting them in the fort, so as to be used either to the north or south, was in a good state of progress June 16. This was the only change that had been made in the defense up to that time . From the stone fort to Fort Duncan there is a well-defined crest, separating the water flowing into the Potomac above the bend at Fort Duncan from that flowing below. This crest it was determined to make the line of defense . It offers the advantage of affording no shelter for an enemy to enable him to turn the line, and compels a direct attack in front . As soon as the men could be gotten to work for after my arrival, I commenced throwing up a field - works were for six guns about the middle of this line of defense . Two other works were also commenced, one on the left near Fort Duncan, and the second on the right at the foot of Maryland Heights. These works were so located as perfectly to command the gullies leading up from the river and to give a cross-fire over the whole line. About half way up Maryland Heights and above the right field-work is a plateau which affords a good position for flanking the west slope of the Heights, and also for enfilading a ravine that extends in front of the line of defense. A 50-pounder Dahlgren gun was taken from the naval battery and put in position to effect both these objects. It can