How would you try to take over Salamandastron?

[Sanddunes]

Hide some of my army so the hares get overconfident charge me and when that happens ambush them.

[Jarky Thistlebrush]

Give up and live peacefully.

[One-Eye the wildcat]

Boi.

[Nerys]

I'll add my 2¢ to this, even though a vermin takeover of Salamandastron is impossible for several in-universe reasons...but let's try it anyway and see what happens!

But first, some reading:

Thus the highest form of generalship is to balk the enemy's plans . . . and the worst policy of all is to besiege walled cities.

- Art of War, Ch 3:3

"Sun Tzu said that! And I'd say he knows a little bit more about fighting than you do pal, 'cause he invented it!"

But seriously, laying siege to Salamandastron is a bad idea. It's not remotely a city, but among other things, it's heavily fortified and armed with trained professionals, has an endless supply of drinking water (thanks to the events of Salamandastron), and unless you make him/her angry enough the resident Badger Lord will be happy to sit back and turtle, until your army dwindles to the point where they can stroll on out and personally end you.

So, we have to take over the mountain without it ever going into siege mode. Which means initially, it is imperative the mountain does not know your horde exists. After all:

All warfare is based on deception.

- Art of War, Ch 1:18

Here's the best sequence of events I could think up:

0 .) Case the mountain
        Exactly what it says on the tin. "Know your enemy". Without being noticed, scout a way into the mountain that would allow you to get in unseen.
        Best case scenario: You find the secret entrance from Lord Brocktree, and it can be made functional with a little work.

I .) Stir up trouble
        Before we get to step 2, let's divide the badger lord's forces some more (and hopefully get him away from the mountain!). Set up shop somewhere in Salamadastron's sphere of influence—but a considerable march away—and cause a ruckus. Do vermin stuff. The Badger Lord is duty bound to guard the western shores, so they'll come after you. At this point you will have split your forces in half; 'A' horde (led by you) sneaks back to the mountain for step 2; 'B' horde (led by some unlucky captain) stays behind to fight the oncoming forces. You'll want to create a big enough of a hullabaloo so that the Badger Lord decides to join the fight themselves, for best results.
        Best case scenario: The Badger Lord leaves the mountain with ~50% or greater of his/her hares to fight 'B' horde, and they don't know 'A' horde exists. The situation hasn't changed; you still have a 4-1 advantage on both fronts.

II .) Break up, Break in
        Now for the actual fighting! The hares are going to be extra vigilant now that the Badger Lord's gone, so 'A' horde needs to move swiftly and carefully. Use the entryways or paths into the mountain you scouted in Step 0 to get in, and flood all passageways and tunnels in the mountain with soldiers. Just prior to the infiltration, you'll need a decoy band of soldiers to directly harass the mountain; 1: To stop the hares from looking elsewhere while you break in, and 2: To stop the hares from fleeing out the front door once you're inside. Meanwhile, 'B' horde will be engaging the Badger Lord on their own terms; and hopefully they manage to do a bit of damage, however pitiful.
        Best case scenario: 'A' horde takes the mountain with less than a third of their forces lost. There is at least half of 'B' horde left, and they have either won or retreated; either way, the Badger Lord's army has taken multiple casualties and they are on their way back to Salamandastron. All hares in the mountain have been defeated; none have escaped to tell about the mountain's situation.

III .) Deal with the Badger Lord
At this point you've accomplished the goal of the thread, but you still have the Badger Lord and his small band of hares to deal with. However, you have the high ground! Make things look like all's hunky-dory, then once they approach the mountain, rain down judgement from the turrets of their own fortress! Hopefully, what's left of 'B' horde will be right behind them and they'll be caught in a death sandwich.
        Best case scenario: You win! The Badger Lord's dead, and he hasn't decimated too much of your remaining army with the Bloodwrath. And on top of that, you are now the richest vermin on the west coast (remember, badger treasure)! Draw a gilded cup of October Ale imported from Redwall seasons ago, and cut a jig using dance moves that won't be invented for another 750 summers.

Disclaimer:
        This plan would never work, and not just because I thought this up in a few minutes after seeing a few pages of The Art of War, or because there are too many variables and everything needs to go right, or that no vermin horde ever would be competent enough to pull this off. All that the spirits of the ancient Badger Lords or Martin the Warrior have to do is snitch on you, and that's it. Game Over. So I believe Jarky had the right idea when she said:

Give up and live peacefully.

[shisteer of nothing much]

Yeah, I think Jarky has the best chance of survival. (Though she still commands a horde of vermin so her chances aren't that great) I think, once I was in Salamandastron, I'd take anything of value, build a couple of ships and take to the high seas to terrorise other areas.

[IronBoomer]

Pull an Ungatt Trunn, have a massive horde, but wait until the resident Badger Lord dies off or is otherwise engaged.

Treat your troops wisely so they don't leave.

Preform a comprehensive search of the mountain and make sure to post at least a guard on the basement entrance; fortify the mountain.

Be generous with the locals- you want to win their loyalty.

When the Badger Lord comes back, use a psychological warfare game to make the resident Badger Lord or Lady seem more crazy than Cregga.

If all else fails, have an escape ship.

[shisteer of nothing much]

I think you'd need an escape ship even if it goes well. Bad guys never manage to keep the strong fortresses so it might be better if you grab all the valuables and such and run. (Or float)

[Booklover]

Diplomacy is always the key. I'd disguise myself as a hedgehog, then get my two hundred vermin to chase me and try to kill me. When the hares and the badger see, they'll come to my rescue and get me safely inside the mountain, having their numbers thinned slightly in the process. Once inside, the answer is simple: use iocane powder to poison the food and water. Once everyone's dead, open the gates and let the vermin in.

Iocane powder? Prepare to die.

So what you do is learn how to time travel, and find a time when Salamandastron has been  deserted. Then, once you've taken it, you block all entrances (you have enough water and food in Salamandastron, as somebody pointed out), and post competent guards at any possible places where the returning hares will force entry, and make sure you have plenty of traps. Of course, this is purely theoretical as even in Redwall (the series), you can't, as far as I know, time travel.

Another option is to give up and go home. Then, you survive, the hares and Badger Lord/Lady survive, some innocent woodlanders may be harmed by your army, but hopefully you can prevent that as much as possible. Everyone's happy!

[shisteer of nothing much]

What's wrong with iocane powder? It's a logical solution to the problem.

Do we have to figure out a way of keeping the fiery mountain or do we just have to capture it? I'm not sure the former option would be possible.

[Booklover]

What's wrong with iocane powder? It's a logical solution to the problem.

Do we have to figure out a way of keeping the fiery mountain or do we just have to capture it? I'm not sure the former option would be possible.

So it wasn't a reference to The Princess Bride? 'Prepare to die' is a quote from The Princess Bride. 'My name is Inigo Montoya, you killed my father. Prepare to die.'

[shisteer of nothing much]

Well... iocane powder is from Princess Bride but I wasn't referencing it, I just took the opportunity to borrow a very useful resource from it.

[clunylooney]

Oof! Salamandastron is way harder to conquer than Redwall, but I'll try.

Well, I would make sure that the hares and badger lord didn't know I was there. I would make my camp as secretive as possible, but close enough to Salamandastron so I could make quick attacks. Maybe, I would kill all of the bats at bat mountpit and conquer that, then use it as a base to further unfold my plans. I would set up a camp of 100 of my vermin right at the base of the mountain, if any hare tried to get out, they would be met with 100 vermin at their doorstep. I would have my second and command be in charge of this brigade. I would not be present there. I would send out my remaining army at bat mount pit to go out on recruitment parties, each with 20 vermin. I would still keep 20 vermin with me for protection. I would wait for the 100 vermin at Salamandastron to die off. I am not dumb. I know that, while my vermin will kill many hares, only 100 vermin against 50 long patrol hares and a badger lord would be suicide. Let's say that my horde at Salamandastron kill about half of the hares with surprise attacks and all out warfare. Let's also say that they kill almost all of my vermin including my second in command. If any vermin who live make it back to bat mount pit I will kill them because they mustn't know that I tricked them. Now the hares number about 25 or 30 and they think that the threat is over. I also now have gained from my recruitment parties, gained another 200 vermin. That makes my army number 300.
While the hares and badger lord are having a victory feast, I would secretly sneak in to any window/opening I could find with my horde of 300. I would cut off and escape routs to for the hares except for one. I would then surprise the hares and kill them. But the badger lord is still a problem. So, I would taunt him myself and lead him through the one escape rout. But it's a trap! I have 20 of my best spear throwers at the corner and when I run by, they will throw 20 spears at the badger lord. If that didn't kill him, (but it probably did) I would have set up my new second in command with 20 more beasts at the next corner. They would go into hand to hand combat with the bleeding badger lord and while they were fighting, I would grab a throwing spear of my own and throw it at the badger's head.

That is how I would conquer Salamandastron.