Spoilers! A Cold Dragon against a castle?

[Corvo]

TL;DR: If you were a cold dragon, how would you attack a castle?

In my DoM campaign the kingdoms of Dale and Erebor managed to re-open the Old Forest Road.
They (re)built three forts along the road, every one garrisoned by some two-hundred men.

So, comes 2960, and the Forest Dragon awakes. In my campaign the beast is kept in thralldom by the Nazguls through the Chain of Thangorodrim from Tales from the Wilderlands.
And (in my campaign) the Elven Path is all but impassable, thanks to the Werewolf, so the Dragon is going to be unleashed on the Old Forest Road.

Now, a firebreathing beast is well suited to attack towers and castles, but how would the Forest Dragon defeat the garrison of one of these forts?

[Rich H]

Now, a firebreathing beast is well suited to attack towers and castles, but how would the Forest Dragon defeat the garrison of one of these forts?

Starve them out.
Persuade them to leave.
Ambush and destroy patrols.
Use minions.

I wouldn't risk my beautiful dragon hide in any kind of direct assault is what I'm saying, unless I was absolutely assured of absolute victory; Smaug's death is far too recent in my mind to make the same mistakes as he did.

[Finrod Felagund]

Other than what Rich is saying (especially allying the Dragon with an Orc Legion), the only thing I suggest is that you give the Dragon fiery breath.

[bluejay]

Could the dragon not sneak up the walls at night and simply let loose inside one of the fortresses? If they're not expecting an attack from it then I think that would be particularly effective on the first fortress.

[aramis]

TL;DR: If you were a cold dragon, how would you attack a castle?

If the castle is river-watered, deprive them of water. In a couple days, they surrender.
if well watered, drop rocks upon/into the well.

Frostbite is a highly effective antipersonnel attack, one that leaves the walls standing nicely.

Frost-heaves and jacking effect can render walls unstable; night-time raids to freeze the same section every night (with daytime thawing) can make for quite unstable wall sections.

[Falenthal]

I must admit this thread has had me wondering for a few days. Yes, indeed, how does a great beast without a fire-breath attack a well defended and hardy stronghold? I don't know, really.

I should read again the description of the Forest Dragon to see how cunning, charming or intelligent it might be. But my fuzzy memories seems to recall that the Forest Dragon was more of a near-mindless beast, driven by hunger and greed, but not prone to scheming plans and allying with anyone, even for a mutual interest. But this might be my memory.

What I can come up with is that such a beast would probably do a first all-out-attack on the stronghold. It is accostumed to win every challenge by brute force alone. After this first attack, that would be repelled by the defenders (with some casualties and probably some structural damage to the hold), the Forest Dragon will turn a bit more conservative: it will stalk the Forest around the stronghold, and ambush and hunt every humanoid group that approaches or leaves it. Time is on its side, and it knows that by doing so the inhabitants will starve and start going out in hunting parties as a desperate measure. They will be easier to be hunted this way.
At some point, the captain of the garrison will order a last resort movement: different groups will go out of the hold at the same time, in different directions. The dragon won't be able to hunt them all, and some should be able to escape and ask for help.

The heroes might be inside the stronghold during the one-beast-siege. Or they might be one of the hunting parties. Or they might be the ones that find one of the exhausted messengers, with a plea for urgent help against the dragon.

[Otaku-sempai]

As Raenar from Tales from Wilderland is a Cold-drake, we could examine how he might approach a keep. His Thick Hide would protect him from most assaults while he brings down the gates with his Great Size and Horrible Strength. Those who try to confront this Thing of Terror in close-combat will have to overcome his Foul Reek. Raenar would attack groups of defenders with his Poison Blast, and anyone who survives that would probably be ripped to shreds by his Savage Assault or daunted by his Dreadful Dragon-spell. And that is when the dragon isn't simply Biting or Rending anyone he can reach. The lack of a flame attack doesn't seem like it would hamper him much. Raenar has slept for several centuries, recovering from wounds he received over three hundred years ago. His size and strength are diminished; imagine what he would be like if he was fully recovered.

The Forest Dragon, by contrast, is one of the Long-worms such as Scatha. She is noticeably less powerful than Raenar, but she has also been diminished by her long hibernation. The Forest Dragon is without a breath weapon; however, I do not know if that would be typical for one of the Long-worms.

[Falenthal]

Upon reading the description of the Forest Dragon in page 99 of Darkening, it seems that she will probably use her Dreadful Spells: Dragon Spell upon the garrison, probably in a series of daily attacks, aiming to reduce the number of warriors as much as possible.
Having her hide in the surrounding Mirkwood and act as Horror B-Movie monster if a hunting party leaves the stronghold might be fun, too.

[Otaku-sempai]

Upon reading the description of the Forest Dragon in page 99 of Darkening, it seems that she will probably use her Dreadful Spells: Dragon Spell upon the garrison, probably in a series of daily attacks, aiming to reduce the number of warriors as much as possible.
Having her hide in the surrounding Mirkwood and act as Horror B-Movie monster if a hunting party leaves the stronghold might be fun, too.

Okay, although the Forest Dragon is not a Cold-drake--unless Corvo makes her one, in which case he should probably give her the same abilities and attacks as Raenar (see Tales from Wilderland). Foul Reek and Poison Blast would certainly add to her repertoire, and she should probably retain her Special Ability: Mirkwood-dweller. However her Dragon-spell does not kill, and will not have a cumulative effect of reducing the fortress' manpower unless it saps the morale of the defenders and causes them to forsake their duties and desert (is that what you meant?).

[Falenthal]

However her Dragon-spell does not kill, and will not have a cumulative effect of reducing the fortress' manpower unless it saps the morale of the defenders and causes them to forsake their duties and desert (is that what you meant?).

Failing the Corruption test against the Dragon-spell paralyzes the victim until the dragon is dead, or a powerful Wizard uses his abilities to dispell the enchantment. Imagine that, with 10 Hate points, the Forest Dragon could paralyze 5 soldiers (or more) every day. As stated before, time is on her side. Also, this is a forest fortress, not Minas Tirith. How many able men is a garrison composed of? 50? 100? Give her ten days and everyone will be all about cowering and shuting their eyes at any movement coming from the forest.