Primary
First off, you want to talk to Ascalon Guard Hayden who is holding the Torch. He will pass the torch to you and you'll have to carry it throughout the first part of the mission. You need to light the Storm Beacons until you come to Ascalon Guard Tolis (A on the map). You shouldn't have too much trouble against the centaur patrols.

When you reach the second guard and give him the torch, he'll open the gate and you'll receive a new objective, which tells you to unlock the fortress door, which when complete will add: stop the siege by destroying the ballistae. Simply follow the path. Grab a Powder Keg from the nearby Dwarven Powder Keg Station and blow the gate open when you reach it (B on the map). Here you'll meet your first two ballistae, which are guarded by a boss (point C on the map). After defeating the mob, run to the left and hit the lever to access the next area. (To activate the bonus, run further left and use a keg to blow open the doors where Rornak Stonesledge is imprisoned.)

Note: If you drop a Powder Keg at any time, it will explode and you will have to get another one. The explosion does not harm you, your allies or enemies.
Tip: Try and clear all the monsters first, then use the Powder Kegs.

The next part is straightforward. Fight your way through the enemies to break the siege. Destroy the two ballistae as well (D on the map). You'll meet up with Rurik and go through a short movie, where King Jalis Ironhammer will give you an Enchanted Torch. Run across the bridge and turn left and you'll eventually get to three storm beacons guarded by Summit Beastmasters (E on the map). Light these three beacons to complete the primary objective.

Bonus
To get the bonus, free and speak to Rornak Stonesledge, the Dwarf at point 1 on the map. He will ask you to slay the Ice Drake.

The Ice Drake is Whiskar Featherstorm. His cave is made accessible using the dwarven Powder Kegs. Bring a keg and set it up against the planks of wood that are directly behind and to the left of Rornak (Rornak's left, that is). Once this barricade is destroyed, there is nothing to hold the snow blocking the pass, and it will slide down the hill and out of your way. If it does not melt away, bring more kegs and place them.

There are only Frostfire Dryders on the way to the Ice Drake, and the beast itself is not too taxing (2 on the map). The dryders can place a strain on healers if not tackled effectively, as they cause collective group health degeneration with the Suffering spell.

NOTE: You cannot get to the bonus without lighting the "storm beacons" first. So be sure to grab the torch when you start. Also, you can't get the bonus without killing the Frostfire Dryders surrounding Whiskar Featherstorm, which means you can't lay all attention to him.