Valve’s released source files for some of the official Team Fortress 2 maps, as detailed in this blog post.
Maps included are Lumberyard, Ravine, Badlands, Dustbowl, Granary, Gravelpit, 2Fort, Badwater, Goldrush, and Hydro.
For aspiring map-makers, this essentially allows you to edit existing levels instead of having to start from scratch.
Full thing through there.
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