First, this is a COLD map. This means that your mech will run cooler, your heat sinks will be more effective, and so on. You can fire a little more often without overheating, so it's a great place to go for max brawling firepower. If you're in an energy-weapon-heavy build, then be glad you dropped into this map. It's among the most forgiving of laser boat alpha strikes.
Second, the day and night versions are essentially the same. Topographically, they ARE identical. While spawn points vary a little from one mode to the next, they are in the same general area. The DAY version has some serious haze, which cuts unaided visibility significantly. And because it's DAY and snow is WHITE, night vision mode is washed out. This is a GREAT map for using thermal vision mode. Night version is fine unaided.
On to the features!
JENNER ALLEY! Starting from cap GAMMA, go straight up a little, then hook a right toward the center of A3. Follow this to THETA. From there, cut straight down. Everything to the right is included, though you're best to stay as close to the sheer cliff on the left as possible, to avoid getting sniped from above. This goes straight down the 5-line to the edge of the map, and follows it down-left and left to just below SIGMA.
So named for being the route of choice for fast scouts, Jenner Alley is the site of many a wild light mech dogfight. Slower, heavier mechs would usually stay above and let the lights go searching for the enemy by taking that long way around, along which they could stay out of LOS to any mechs above. In ASSAULT mode, it's not uncommon to have mechs run the length of Jenner Alley to the enemy cap to try to win by cap (detestable way to win, IMO). A little tougher in CONQUEST, as THETA tends to come under fire from whoever occupies the ridge directly left and down-left from it. Tends to be largely uncontested in many SKIRMISH engagements.
Slow, heavy mechs should avoid being down there. Speedy mechs that dare should remember that they ARE fish in a barrel if observed from above, and speed is their best ally.
THE TUNNEL! C2-C3, and the top-left corner of D3. This is the shortest and fastest route between GAMMA and SIGMA, which are basically the ASSAULT and SKIRMISH starting points as well as the CONQUEST start caps. It's impenetrable to air and artillery strikes. Barely wide enough for two mechs to pass shoulder-to-shoulder, it's the site of many a hilarious collision between opposing mechs sprinting to surprise one another's forces.
There's not much room to turn around in there, and there ARE a couple small structures to further hinder maneuver. They, along with the curves, provide some decent cover for trading with enemies, but not that much. LRMs are useless in here--low ceiling. Also note the middle entrance in the upper-left quarter of C3. Used to be popular for a sniper to pop out there and attack the force that spawned the upper side. Then seismic sensor came along, and now such a sniper usually ends up losing a trade with 3-4 kill-hungry enemies. If you catch the enemy unaware, though, you can make an ERLL Raven do some work from here. If on the side that spawns top, at GAMMA, you can possibly score some C-Bills watching it and popping whoever is foolish enough to try it. One important thing: If you go TUNNEL RUSH, you had better commit to it and rush well out of the tunnel. If you get stalled at the tunnel mouth, on either side, you will likely get focused into oblivion fairly quickly. It's a bottleneck. And thus, it's a gamble.
THE DROPSHIP! Left-to-right across the middle of C4, you see a cut. In the middle of that is a crashed dropship. If you look inside, you'll see the carcasses of the mechs that were lost in that crash. Creepy. It's at the crest of a hill, at the left end of it, across which much trading, hill humping, and poptarting takes place. At the right end, there's the ship's tail's vertical stabilizer for some cover. The ship is broken in the middle, offering some corner-peeking to either side.
Most brawls used to set up on either side of the ship. Those lights that ran Jenner Alley? They'd pop up behind the enemy on either side and spot for LRMs. Trades happened, then the hill humpers and pop tarts would start trying their luck at the left end of the ship. Sooner or later, one side would grow a pair and push over the hill. It would devolve into one big brawl. Any mech that pops up on that hill, WILL get focused by the enemy. This is not a good place for lights.
OTHER: The buildings on either side of the dropship make for great cover, corner peeking, pop tarting, and some interesting chase-brawling. Also, manage your speed wisely in there. It's all well and good to be in a fast mech, but you want some margin in case you turn a corner and come face-to-face with another mech. Rubber-banding and collision damage happen a LOT in between those buildings. A lot of brawls nowadays happen further to the right, along the B4/C4 line and in the upper-left of C5. It's not hard for the team that spawns SIGMA to sneak in there, execute a left turn, and push HARD into the force that spawned GAMMA. Disciplined corner peeks and hill humps, with some quick twitch-style alpha strikes, will do well here. Stay mobile, though. DON'T get comfortable behind a particular building or terrain feature.
That's it for the standout features of FROZEN CITY. Again, looking forward to the feedback of more talented veteran players.