Route Kanal is the network of canals, sewers and maintenance tunnels leading into and out of City 17. The water is polluted in several places, making it harmful to the player and enemies, and clean water is also found. There are many rebel strongholds, as the canals are being used as an escape route for many besides Gordon. At one point there is a large dam, where a nearby waterworks facility houses Black Mesa East.
Much of the canals are light concrete and water-stained, with interior areas a darker concrete. There are occasional metal pipes the player may be able to enter. The entire area should give a heavy feeling of disuse, but not disrepair- walls should not be crumbling, but can have plenty of graffiti.
The canals are littered with large amounts of trash and debris, much of it metal or rock. Large metal containers could house rebel stations, as can any similarly-designed safehouses. These structures are inconspicuous, and are generally built out of whatever material is already there.
In the latter part of the area, the canals open into an outdoor river. The design here should change to reflect that, with natural barriers instead of concrete. This is also the portion where the airboat is used, so these sections can be wider, longer, and incorporate more interesting traps and tricks (such as jumps).
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.
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 commonly in the canals, both in tight areas and under bridges. Consider how difficult the barnacles will be to either avoid or defeat when placing them; nobody wants to get sucked up by one in order to kill it.
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 miniaturised, 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.