Hydro
Introduction
This article will go through the design theme used in 
Team Fortress 2 maps such as tc_hydro, and cp_powerhouse, among others. This will be mostly focused on Hydro in specific, unless stated otherwise.
Environment
Location
This theme typically has a balance of indoor and outdoor areas (with exceptions, such as ctf_turbine). The outdoors of these maps may be set in the dusty, arid deserts similar to cp_dustbowl and pl_goldrush. However, its unique architecture greatly sets it apart from the dirty, worn-down buildings in said maps.
Set inside a hydroelectric power plant, featuring heavily industrialized concrete buildings. The bright, clean walls with many windows, combined with bright sunlight, create inviting and highly visible environments, which works great for gameplay.
Textures
Textures used in external parts of buildings consist of neutral, white, concrete/brick walls, such as concrete/concretewall011/012 (along with variant a), and concrete/concretewall015 (along with variants a and b). In tc_hydro, buildings lack team colors due to the nature of the gamemode, since RED and BLU are constantly fighting for control over them. In other maps, such as cp_powerhouse, the two bases feature team colors. These textures are still the familiar bright white, with a team-colored stripe at the bottom of a layer of the construction.
Architecture
Spytech
The last objectives of tc_hydro, also heavily feature Spytech.
Gorge
A similar map theme to Hydro can be found in cp_gorge. The map is set in an alpine water purification plant. Due to its alpine theme, its surroundings are lush greens on gray rocks, compared to the sandy yellow rocks of Hydro. Moving up to the first capture point, there is a giant mountain reaching up towards the sky found to the left of the point. On the other end, a "gorge" which is too small to be considered a gorge.