Next time you see a craftsman building a wall brick-by-brick, remember that his days are numbered. No not by the Terminator, but by TERMES, a project to create a swarm of builder robots.
The idea is simple in principle, but difficult to get right. Take a group of autonomous machines designed to be able to clamber over structures of bricks. Each robot carries a brick to the correct spot and deposits it. As long as the swarm is functioning under the right algorithm, eventually whatever you want to build will be built.
The Self-Organizing Systems Research Group at Harvard has built demonstration robots that can perform swarm-based building. The robots used are simple but specialized for the task. For example, in place of wheels they use "whegs" (WHeels with lEGs) which have hooks that allow them to climb over steps in forward or reverse. The don't have sophisticated position sensors, no GPS for example, but they can detect the special blocks using IR sensors and a tilt switch tells it when it is level.
Using just these simple inputs the robot can move bricks around to build complex structure - see the video for an example of a castle:
It is important to realize that there is no central control for the robots - they all just get on with building the plan. The algorithm is based on a "Structpath", which is a path through the bricks that all of the robots follow. The rules that the robots follow guarantee that the structure will be completed and untraversable paths, deadends and deadlocks are avoided.
If you would like to see the algorithm in action it is illustrated in the following video:
At the moment the suggestions for its possible use include building sandbag walls or assembling structures in dangerous locations such as space. Given the recent increase in freak weather and flooding, perhaps the sandbag idea is the one to go with. If each city has a few cheap and tough sandbag robots it could presumably be set to build defenses quicker than a human team could manage.