The number of guns captured by the allies at Sebastapol was 3,839. Of these 2,087 guns were mounted in the defences:
8-inch: 61 (i.e. 64 pdrs)
7-7.5": 87 (i.e. 48-56 pdrs)
6-6.5": 981 (32 pdrs)
5-5.5": 395 (24 pdrs)
smaller guns: 491
13" mortars: 18
10" mortars: 8
Brass Cohorn mortars (6-6.5"): 21
Brass field guns: 16
Wall pieces: 9
Another 112 brass field guns were captured, and the 1,640 guns were largely on the ships.
In April 1865 there were positions to mount 1,120 guns and mortars in the Washington defences, and 807 guns and 98 mortars were actually mounted (905 pieces). In December '62 643 guns and 75 mortars were mounted on a 37 mile front.
Washington in 1865 (the most armed the defences ever were) had 905 pieces on a 37 mile front, or 24.5 pieces per mile.
Sebastapol had 2,087 pieces on a frontage of ca. 4 miles, or ca. 522 guns per mile. That a firepower density more than 20 times Washington.
Umm...No.
The city of Washington was impregnable in 1864.
Absolutely not. Had Early attacked a day earlier he would to waltzed straight through the defences and captured Washington. Due to the long length of the defences, there were relatively large distances between the forts, and they were not mutually supporting. You simply had to overwhelm a single fort and you'd penetrated the defences. Due to a "crust" defence being adopted north of the river, there were no further points of resistance.
In June '64, Grant had stripped the defences. North of the Potomac were only 3 infantry regiments, 2 Veteran Reserve regiments and about 2 regts worth of artillerymen. On the 10th June, had Early known how weak the defences were he could have walked straight through them. Only the timely arrival of elements of the 6th and 19th Corps saved Washington, and it's a good thing Halleck overruled Grant and pulled them in.