Route Kanal is the network of canals, sewers and maintenance tunnels leading into and out of City 17.It is mostly filled with water, however many parts of it are polluted with toxic waste. There are many rebel hideouts and weapons caches, as the canals are commonly used by rebels to escape the city.
Much of the canals are light concrete and water-stained, with interior areas a darker concrete. There are also a few metal pipes that the player may be able to crawl in to. The canals should appear to be in a state of disuse: graffiti adorns the walls, the ground is littered with trash, etc. The canals are not, however, in a state of disrepair, so walls should not be crumbling and wild vegetation should not dominate the dry areas. The canals are littered with large amounts of trash and debris, much of it metal or rock. Large shipping containers could house rebel caches, as can any similarly-designed safehouses. These stations should be made out of materials that were already present in the canals and should be somewhat inconspicuous to avoid detection by the Combine.
Areas should feel tight and linear. Although Gordon's passage through them happens during the day, they can be traveled at night with just as good visual effect. However, this may reduce any lighting contrasts between inside areas and open, outdoor areas.
Most of the traps that the player would come across are set up by the Combine, and should deter the player rather than help. There can be an occasional trap set up from a rebel stronghold, but the majority should be set against the player's advancement.
The most common sounds should be sounds of pursuit. The canal levels have a feeling of escape (amplified during the airboat sequences) that is aided by the sound of pursuing helicopters, gunfire, and other hostile noises. The canals themselves produce little noise beyond the lapping of water, quiet sounds of wildlife and a slight breeze; further inside City 17 itself, city sounds and PA system announcements may be heard.
Some areas can be quite disturbing to be in, such as a sewer filled with the charred corpses of rebels that were killed and hidden by the Combine.
The Combine makes its first strike against Gordon in the canals, so a good number of armed Metropolice should appear throughout. They can appear in the canal itself, on streets above, or as reinforcements flown in by dropships or driven in by APCs (armoured personnel carriers). The Metropolice also use traps and ambushes when possible, so there is potential for scripted scenes.
Barnacles also appear quite frequently in the canals, in tight corridors and under large bridges, usually in large clusters. Consider how difficult the barnacles will be to either avoid or defeat when placing them, as it can be frustrating to have to be captured by the barnacle to be able to take a shot. Rebels have set up an underground 'railroad' throughout the canals, which helps Gordon considerably. Consider creating safe havens where rebels are (or were) stationed to give the player a break from combat, but also remember that the Combine are actively moving in to close down the rebels' activities; a miniaturized, Alamo-style siege is a definite gameplay possibility.
While no striders appear in the canals in the game, they are not restricted from entering, should they be desired. Antlions simply do not go near the canals. Headcrabs are best limited to tight sections of a Route Kanal map; Headcrab shelling has taken place around the periphery of City 17 in order to disrupt the rebels' supply routes.