Shattered Colony: The Survivors
For one, you'll only ever get to build four things in this zombie-infested Flash title: barricades, sniper towers, supply depots and workshops. You need to consider your placement of each thing carefully; supplies and ammunition need to be delivered on foot, so place a sniper too far from base and they may become overwhelmed before a runner arrives with assistance. You'll also have to place workshops at the entrances to buildings to scavenge any supplies they might hold. Icons at the top of the screen display all the survivors, boards, and bullets you have, and building something (done by clicking the appropriate icon in the bottom left) requires a certain amount of each resource.
Technically speaking, each of these constructs can be upgraded. You can put more people in the workshops to make things go faster, add boards to the barricades to make them stronger and increase the range and ammo each sniper tower holds. And that's pretty much it; you're not going to get anything else to work with. While other games might throw a variety of mutant zombies at you, Shattered Colony only has the slow, shambling and stupid variety. Holed up inside abandoned buildings and scattered throughout the city, zombies will not approach you unless you engage in construction too close to them or fire randomly in the immediate vicinity. It's all about holding out as long as you can, and learning to construct your network of survivors for your best advantage.
In some strange way, the spartan features are really what make the game shine. Shattered Colony doesn't want you to craft the world's most impenetrable maze of defenses. It wants you to survive. Everything in Shattered Colony is finite. (Except for the gameplay, considering the presence of a map editor.) Each decision you make is vital. Do you waste those bullets picking off that lone zombie or wait for it to arrive in a pack with everyone else? Do you invest everything in one sniper tower or are you willing to risk building another two? Each choice you make is likely to come back to haunt you. Strangely realistic and visceral in a top down, cartoony sort of way, there's no better feeling than when you've finally survived to the end. Of course, there's nothing more depressing than realizing you're out of bullets when the zombies finally break down your barricades.