Nova Prospekt is the prison area in Half-Life 2 which has been "renovated" by the Combine to store human prisoners and convert them into stalkers. The player leads a swarm of antlions into the complex and deals a crippling blow to the Combine, which triggers a revolution that sweeps City 17.
Being a prison, architecture is appropriately spartan. Exteriors use bog-white plasters and concretes, while interiors are covered in peeling plaster and tile. Many materials are often used repeatedly in different settings throughout Nova Prospekt. There is also a motif of "restriction": prison bars, heavy metal doors, and fences reinforce an ominous feeling within the player.
Patterns are particularly important in this setting: Long arrays of cell doors, exposed pipes, railing, or architectural elements (such as columns) complement the sparse decoration. However, this motif extends far beyond appearance; the level layout actually resembles cell blocks. At the end of each cell block, there are usually sliding doors, controlled from a nearby security station. Players learn to accept this pattern and it only enforces the prison theme.
As always, bright teal spotlights betray a strong Combine presence. For the most part, lighting is either natural (a subdued grey), Combine (a bright teal), or yellow-orange. Very little of the outdoors is seen, because of the indoor nature of the prison.
Because of the layout of Nova Prospekt (and prisons in general), a level set in the environment can feel very monotonous, making these levels difficult to design. Fortunately, when something is different, it has a larger effect on the player.
Because of the abundant teals and greys in the lighting, textures should be equally subdued. Red and yellow textures will jump out significantly. Again, depending on your design this can be a benefit or problem. Also, use worn textures and props; the facility's upkeep is obviously not a concern.
The prison is effectively silent, save for occasional sounds of distant gunfire and fighting provided by soundscapes. This allows the place to feel scarier, as well as empty. If you do wish to use sounds, use those associated with the Combine for the best effect.
Since the player turned off the thumpers on the coastline, tables have been turned against the Combine as endless swarms of antlions arrive to aid the player (since he now has the bugbait with him). There should also be areas of the level where the ground is especially soft, providing a location for antlions to appropriately spawn from.
Combine soldiers should make up the bulk of the opposition in levels of this area, along with ground turrets. Turrets should be placed so that they either stop antlions or are easy for the antlions to destroy.
Helicopters are fitting for battles in the prison's courtyard, along with gunships. Striders would not be fully appropriate for a target as small (physically) as the player. Zombies, headcrabs and other fauna may be found in the bowels of the facility, but not in any areas of active use by the Combine. Allies are right out.