Well, actually DC could be a solid and very smart idea. First, a few facts about the "walkers".
1. They aren't smart, they can't strategize; they will mindlessly walk forward, even onto spikes or spinning blades that are quite obvious, in search of food.
2. They aren't strong enough to break solid stone; even in large groups a thick stone wall is a great defense.
3. They can't jump or climb, the effort is simply too complex for their brains to coordinate.
From here, let's look at the pros and cons.
Pros
1. Stone Buildings As anyone who has been to DC can tell you, there's a lot of stone and brick buildings, sure there are still windows, but you can board or barricade those up on the first floor and be fine. A lot of buildings are also surrounded by stone and metal walls; barricade the gates and put up spike walls and you can hold a building quite well against a zombie horde. As a bonus, many houses are right next to each other, knock out a wall in the upper floors and you can connect multiple houses with one entrance and brick over the rest.
2. There Could Be People! Perhaps a remnant of the US government that is benevolent, or a group of friendly (not Terminus friendly!) survivors that have set up some defenses exist there. DC would be a natural place for people to go to.
3. A River Runs Through it Technically two rivers run through DC, but I liked the title of this one better. The Potomac and the Anacostia would provide relatively clean drinking water for survivors in DC.
4. Mazelike Streets As anyone driving DC can tell you, DC can be a confusing maze of streets and alleys, particularly in certain neighborhoods. It would be pretty easy to set up ambushes and choke points, both for the living and the dead.
5. Mild Climate Relatively speaking, DC has a fairly mild climate. It can get a little hot and muggy in the summer, but nothing like Georgia and while it might get some snow, it doesn't get plastered like the north east.
6. Guns and Tanks There's a lot of military tech near the nation's capital, powerful vehicles, guns and ammunition are all relatively close by. The abundance of security forces, both national and private, also mean lots of small arms and ammunition as well.
Cons
1. It Could Be Swarmed Atlanta was home to massive hordes of zombies at the start of the series, if the horde of zombies haven't dispersed the way Atlanta's has, the group could walk into a major problem.
2. There Are People! Either a dictatorship formed from the remnants of the US government or a bunch of survivors have taken DC and they are hostile to Rick and company for whatever reason. With the local firepower and home field advantage, they'd be a stiff challenge for the group.
3. It's a Crater DC is bombed out, or the air and water poisoned, by attempts to eliminate the zombies. There's no shelter, no food or no clean water for the group to utilize.
Look at the cons and you see, well those aren't much different than any other set of cons in the world of the Walking Dead. Even the prison and Woodbury had hordes of undead that would show up from time to time and swarm them, and obviously the group has gone to several places that are either filled with hostile people (Terminus and Woodbury) or false hopes (CDC or Richmond). Without any direct knowledge of the comic books, I'd guess that the group won't even make it to DC, so it's sort of a moot point; but DC is a pretty strong candidate as a fairly defensible place.
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