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My players are setting up to defend a village from a (small) invading army and they want to have some form of explosives, most likely would be barrels full of pitch or the like, placed under the bridge into town so they can blow it if things get too out of hand.
They've also got a cart, with barrels of pitch, just down the road too, so they can easily thin the herd.

From page 15 of the Scales of War: Rescue at Rivenroar (not what I'm running, it's just what I've found in relation to blowing things up) adventure I have found this;

If the casks on the back of the wagon take at least 5 fire damage, they start to explode. The casks in one of the 4 squares in the back of the wagon detonate each round. Each explosion is a +6 attack against Reflex defence in a burst 3 area, dealing 3d6 fire damage (half on miss). When the first casks explode, tell the players that it’s clear that not all the casks have exploded yet, and that the wagon is afire.

which I plan to use for the cart, however I'm unsure of how to translate that into blowing up a bridge.
I know the DMG has a section on page 65 about damaging objects but the way I see it a stone bridge could have up to 400HP so using that method seems like a bad idea.

I almost feel I could just say they blow up the bridge with something like a TNT style push level and fuse once they retreat back across it, since the first battle is initially minion spawning with a few standard monsters spawning every few rounds it would be an easy way to grant them a short rest, but I want it to feel like there's a bit of pressure on them and that there is a chance they won't be able to do it the first or even second time.
Think the classic movie trope where the lighter doesn't work until the very last second.

And so I have come here to see if anyone has other ideas or can direct me to an adventure that already has this covered.

Note: none of my players really have any ranged attacks (there's only 3 PCs) so they have village minions with shortbows to, at least, blow up the cart with flaming arrows.
Also if it helps the bridge is only about 20' wide and 35' long.

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  • \$\begingroup\$ A key thing to remember about dnd-4e is that it steers pretty hard away from being a simulationist game. If/when you need to mechanize things, aim for fun first, balance second, realism third. You want just enough realism that your players don't get dropped out of the fun. \$\endgroup\$ – Simon Withers Nov 8 '14 at 5:15
  • \$\begingroup\$ I can't wait for this question to become a hot network topic and "How should I go about blowing up a bridge?" starts showing up on other stackexchanges. \$\endgroup\$ – Theik Nov 11 '14 at 9:51
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There's no reason to employ a mechanical solution here, go with the narrative one that makes sense.

If you want to have a mechanical solution, employ a skill challenge to determine if the charges are set properly, wired together proplerly, are lit properly, ignite correctly and in sequence, and then use the results to determine how effectively destroyed the bridge is (0-1 failure it's completely gone, 2 failures it's passable, but slowly, and 3 failures it fails to detonate).

There's no need to concern yourself with anything further mechanically about the bridge as the challenge will abstract all of the elements you're concerned about with "do they succeed, and how well."

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    \$\begingroup\$ I think his mechanical problems stem from the fact that mechanics try to picture reality and a wagon of pitch cannot blow up a bridge in reality. Using a skill challenge to counter the fact that it's not mechanically sound seems to be a bad choice. For me, it would be immersion breaking just the same way that introducing lasers or lightsabers would. I'm not against abstracting challenges, but it should be abstracting otherwise doable actions. Leaving out details because the details don't add up is wrong abstraction. \$\endgroup\$ – nvoigt Nov 9 '14 at 8:43
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In most medieval societies, gunpowder or another form of explosives is not known. So literaly blowing the bridge up will be hard to do if you want to stick to mundane items.

What people did in former times to bring down structures was clever use of fire.

  • Pouring flamable liquid over a bridge and setting it on fire might not ultimately destroy a stone bridge, but it will be impassible while it burns and if it takes down enough of the supporting wood and hemp, it may be impassible to anything but single, dextrous acrobats for a long time.

  • When trying to bring down stone structures, people dug tunnels beneath them, filled them with wood and burning liquids (lard worked well) and set those on fire. When the wood supporting the tunnels burned up, the resulting unsupported hole caved in right under the structure. This resulted in large breakdowns in the walls that fell into the cave in.

Both methods require a hero. Pouring and setting alight will need to be done under enemy fire, you cannot just pour liquid and expect it to stay there until needed. The tunnel can be dug by minions before the battle, but a fire in a tunnel is hard to keep burning, it needs fresh air. If nobody were pumping air into the tunnel, it would burn very slow and may even go out. And the enemy will probably notice this and try to prevent it.

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    \$\begingroup\$ I could have sworn he wrote "I have come here to see if anyone has other ideas". \$\endgroup\$ – nvoigt Nov 8 '14 at 13:57
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    \$\begingroup\$ @waxeagle In addition, this is the adventure. Probably not even the one the OP is playing, just one he found mentioning any kind of explosives. And even there, blowing up a bridge with explosives is NOT written into the adventure. \$\endgroup\$ – nvoigt Nov 8 '14 at 14:19
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    \$\begingroup\$ Water mages are effective against bridges, too: turning the creek below into a raging torrent for a while won't get picked up on as dangerous by your average mook, yet, scouring the soil out from under the abutments and piers is one of the most devastating things you can do to a typical small bridge -- am I correct in assuming this is a small bridge, not a civil engineering monster with piers sunk to bedrock, by the way? \$\endgroup\$ – Shalvenay Nov 8 '14 at 16:31
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    \$\begingroup\$ I read the question, the "explosives" are "pitch or the like," which is fitting for the era but not terribly explosive. It IS powerful, and quite dramatic. There's nothing wrong with this answer. Explosives similar to pitch (per the question) are not going to have sufficient yield to blow up the bridge. They will however be able to spread the fire, work as an accelerant on the woodworks, and rob the oxygen from the air on the bridge. \$\endgroup\$ – Smithers Nov 8 '14 at 17:14
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    \$\begingroup\$ Side note, a fuel-air-bomb could be made with pitch igniting flour (or coal dust) thrown into the air at the right moment. It may be an improper use of OOC knowledge though; I don't think that explosions were understood until the 1800s. \$\endgroup\$ – Smithers Nov 8 '14 at 17:21
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To keep the action movie vibe, you'll want your action heroes (the players) to be the ones to set off the bomb. Lowly peasants don't deserve that kind of spotlight!

Of course, the players' plan to have the (ranged) peasants set off the bomb makes perfect sense, so play along with it initially. Just try to think of some complication which prevents them from actually doing so when the time comes. Maybe an enemy spellcaster plunges the area into darkness, so they can't see their target. Maybe a flock of flying enemies overwhelms the archers. Or maybe it simply starts raining, and the peasants can't keep their arrows lit.

The players will have to think on their feet if they want to salvage their plan. And it's a good plan; you don't want to appear to be punishing the players for it. Make sure the complication seems perfectly natural and reasonable, and that it's not the player's fault.

Most importantly, make sure the players know the plan can/should still be salvaged. Without the peasants, blowing up the bridge ought to be a lot more risky/difficult, so make it appear equally rewarding. E.g. a massive horde is regrouping on top of the bridge; if the players can set off the bomb then it will take out far more enemies than previously expected.

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  • \$\begingroup\$ This is a good idea, but it's so tangential to the question that I'm not sure if it actually qualifies as an answer. \$\endgroup\$ – DCShannon Nov 10 '14 at 2:53

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