Matt Bird writes all sorts of nonsense, but he dedicates a large chunk of his time to writing game walkthroughs.
Out of the greenery and into the white. The campaign is moving into the north, and for the next bunch of levels, you'll be fighting foes in the coldest regions on the planet. Don't forget to bring your gloves.
Coldstep Mines introduces a new hurdle in your campaign against evil: mine shafts. These tricky passageways will carry foes from one part of the map to another in the blink of an eye. In this case, they might emerge in two different spots: near the halfway point and not far from the end. In either case, you'll want to try to eliminate or soften up your enemies as much as possible before they can enter the mineshafts. The mines are a double-edged sword as well, since you can set up Rain of Fire assaults on these spots and blast through foes as they come out in a group. So long as you're properly upgraded, the resulting line of embers will continue to shred enemies as they emerge.
This level brings in a new enemy in the form of Trolls. These little guys are of average hitting strength and health, but they're able to regenerate their wounds over time. This makes killing them in a hurry essential to your longevity. The level allows you to recruit Musketeers as an upgrade to your Archer Towers, but your Rangers Hideout with its Poison Arrows is still a better choice. It will counter their regeneration abilities and prove fantastic against the Gargoyles that occasionally sweep through the map.
There are several different ways to complete Coldstep Mines, but one of the safest is to use a fair number of Archer Towers, deployed right near the spawn point in the north. These towers can take out most of your enemies as they meander down the path. A Barracks along the way isn't a bad idea for stopping the Winter Wolves that sweep through the course from getting too far away from your Archer Towers, and Reinforcements will prove handy in that regard as well.
Archer Towers won't be as handy later on, unfortunately, as Dark Knights will begin to appear in large numbers. Rain of Fire is always fantastic for dealing with these creeps, especially if it's fully upgraded, but for safety's sake you should line the rest of the course with Mage Towers. These are useful against every enemy in Coldstep Mines. When paired with Reinforcements, they can save your butt if something manages to slip through the mines.
- Coldstep Mines on Hard requires more or less the same strategy, though you'll need to make more ample use of Rain of Fire to clear out everything. Don't be afraid to bring enemies in early - the extra money is helpful, and chewing up the amount of time it takes to cast Rain of Fire again will make the battle much easier. (Just be careful which swarms you summon. Calling in Dark Knights won't hurt too much. Bringing in a wave of fast-moving Winter Wolves is another thing.)
- Boost up your Artillery and this level's Heroic Challenge isn't that much of a challenge. Set up two Artillery in the north, by the spawn point, and erect a Barracks with Reinforcements support that can keep the enemies that pass by in place. Set up a row of four Archer Towers one row south of here to deal with the Gargoyles that go through, as well as any lone units that get by your Artillery, and erect another Barracks and Artillery combo by the exit to stop anybody that gets through the mines. The Trolls will be your worst enemies here, but with proper management and Artillery placement they're not as bad as their numbers suggest.
- Coldstep Mines' Iron Challenge eliminates Barracks and Artillery, but sets three Barracks in the middle of the rows. Upgrade as you feel necessary, though the only one that really needs to be bolstered is the right-most barracks, by the mineshaft. There are a lot of armoured units in this map, making a line of Mage Towers a wise idea; that said, Archer Towers are the essential component for survival. These will help you eliminate the stream of Trolls. At the very least, set up four upgraded Archer Towers in the south after you've moved past the initial stream of armoured units.