The hardest part of the level from all my attempts is defending Fort DuRussy, especially during the second phase where you have the protect it. During the first phase I was able to get the AI to largely avoid attacking it by rushing a general and several extra units up there; it seems leaving other areas less defended influences where the AI attacks on the first phase. On the second phase though the AI just seems to go after DuRussy no matter how many units you have at it, and the fort is so small you can't really fit that many units inside so most of them are getting shelled by cannons and running away. Literally at the last ten seconds of the phase tons of Union Infantry started charging at only two units of men and a general in the fort when it switched to the final part of the level. The final phase of the level was much easier to defend, even though the Union does an annoying sneak attack on the city from the south.
Doing a bit of research I'm glad to know I'm not alone in thinking this level is pretty unfair, it seems like a lot of people ran into Washington like it was a brick wall after doing really well in the battles leading up to it. It's less so the level itself being unfair, it's more so that the game seems to encourage building your army in a way that handicaps you on the final level (focusing your men and money into fewer larger units in less corps). Washington clearly was designed to the point where you need to spread your army out to man as many defenses as possible, with one unit manning the defense and at least one unit behind to shoot down enemies that charge into melee, which can't be done with less corps full of 2500 man units. I'm confident that if I redid all the battles from Gettysburg on and focused on getting 4 full corps of 1500 infantry and a 14 man cannons in each brigade this level probably would have been a steamroll for me. As it stands though I'm glad I only had to redo one battle before and was able to pull it off by the skin of my teeth.