I actually think Chasm is more of a three lane map than Siege or most of the others
Chasm is pretty easy once you realize the three lanes are subway (fairly safe), center (not at all safe) and ruined offices (risky).
Trick to surviving on there is to avoid moving through the center unless you're absolutely 100% certain your flanks are covered by teammates. Otherwise you only traverse the map A-C via the subway, or the non ruined side of the offices (preferably first floor).
The ruined side is more dangerous - the patio above C and the entire street below it are exposed to the offices as well as the area above B. They are all very dangerous and should be avoided if possible.
The ruined side closer to the A side is a bit safer, and should be used to approach B instead of walking straight at it - you're deadly exposed to the offices if you're in the B area.
The worst place to be is near B where there are something like six or seven or eight different crossways depending on how you count them.
The area around C itself actually isn't bad at all, as long as you're careful about what you're watching, you can mow people down easily. Watch out for the occasional person poking their head over the patio that can look down on you!
Same story at A actually, there's a lot of places you can hide out with your back covered and only a few frontal approaches to you, plus few people seem to use the underground pathway that leads to the subway there (good route to sneak either in or out of A to flank anyone camping there, and it lets you reach the ruined offices almost unseen).
