Confined. Build a sandbox prison with 1000 prisoners. It sounds very simple, and thanks to the Steam Workshop it is simple. You can get 4 achievements at once by downloading a prison designed to give you this achievement. I used this prison, which houses 1035 prisoners, and I got the achievement immediately upon loading. Good thing, since most machines probably can't run the map.
http://steamcommunity.com/sharedfiles/filedetails/?id=534610...By downloading this map, you can not only get the Confined achievement for Prison Architect, but you can also get Stone Walls - house 500 prisoners, and Iron Bars - house 100 prisoners. You'll also get the Samuel Norton achievement - make $50,000 daily, and if you sell the prison you can get D.B. Cooper - sell a prison for $1,000,000. Arguably some of the hardest achievements in the game.
After you download the map, start the game and navigate to the Extras menu. Go to Prison Sharing and on the left side it should say "1035 Inmate Prison". Click that, then click play on the bottom right and it will begin to load. That's all you have to do for this achievement through this method.
If you want to get it legitimately, then you will have to make a series of successful prisons and sell them until you have a new plot of land with enough money to build a prison large enough to house 1000 prisoners, and the pc to run that. I've never tested it, but I'd say you probably need $8,000,000 or more, and the most I've ever sold a prison for was $2,000,000. Just keep making and selling prisons and making money from the prisons and eventually you'll make enough money.
Have fun