Balance is a crucial thing for Valve when it comes to Team Fortress. Get a map wrong and there'll be emails, petitions...hell to pay. Thankfully, most maps get it right! Here's how they're made!
Valve have released a couple of videos showing the evolution of a Team Fortress 2 map (Viaduct), from planning to completion, as the team tinkers with not just the look of the map, but the layout and design as well.
The clips are accompanied by an interesting run-down courtesy of Valve's Corey Peters, which basically boils down to: We do a lot more work getting these maps right than any of you have probably ever realised.
This point ain't gonna cap itself! [Team Fortress 2]