In terms of defending a city....

[SangoProduction]

Our enemies are full-blooded Orcs, and our allies are Goblins if that makes any difference.

How viable is it to have a wall of wood (or pitch/tar/bitumen/oil/etc) out in front of your actual city wall, which you'd set on fire when you see the enemy? I was thinking this might buy a bit of time for our archers' volley fire to take out more of them, while denying their front line the opportunity to break in and do damage.

And we also thought it would be quicker than trying to build a wall that's meant to take a pounding, as this would basically be a pile (or pool) of flammables. Would I be correct in this statement?

We believe that they won't have near the archer count that we do, but we are going to confirm that with a scouting run this session. We are 100% positive that they won't have siege weapons.

[Umbranar]

I think a burning wall outside your city walls is a bad idea.
Main problem: Smoke.
You will block the line of sight of your own archers making it so you freely give your enemies concealment of even cover on their approach.
Yes it will take a while to break down the wall but I don`t think that`s an issue. The enemy can hug the wall while tearing it down or even using the free material (assuming they can extinguish the fire) into impromptu shields or mobile cover.

There are multiple examples in history of garrisons chopping down all tries or even removing big rocks near forts. Open field around your fortified positions means vision and easy targets for the archers defending the walls.

[SangoProduction]

I think a burning wall outside your city walls is a bad idea.
Main problem: Smoke.
You will block the line of sight of your own archers making it so you freely give your enemies concealment of even cover on their approach.
Yes it will take a while to break down the wall but I don`t think that`s an issue. The enemy can hug the wall while tearing it down or even using the free material (assuming they can extinguish the fire) into impromptu shields or mobile cover.

There are multiple examples in history of garrisons chopping down all tries or even removing big rocks near forts. Open field around your fortified positions means vision and easy targets for the archers defending the walls.

Derp. For some reason I pictured it as though them just patiently sitting while the archers shower them with arrows.

OK, your statements under consideration: what if we had an invisible spotter who could instantly report in the direction and distance that the archers would need to fire?

Should we instead use less solid materials, like oil or pitch, to get rid of the potential cover?

[kalos72]

I would remove the fire part all together.

Just make it a ditch with punji spikes and such. The idea will be to slow them down and force them to deal with the obstacle while your archers pound them.

A wall of any sort will provide a limited amount of cover, something you NEVER want to give the assaulting team.

Make sure to remove the dirt from digging the trench too...they can hide on the far side.

[SangoProduction]

OK. Will do. Thanks

[Umbranar]

Fire could work if you drench the ground with oil/something flammable. Thing is though, the enemy must not know else its useless. In d&d this is hard as a smart enemy will use many divinations to check for such surprises.
Best tactic is to slow them. Ditches with punji sticks work, thick wooden poles with barbed wire, tons of caltrops.
Heroes of Battle has some good mundane suggestions.

[Lerondiel]

What's the scale of the conflict? hundreds of orcs..thousands...tens of thousands?

[Sagetim]

Our enemies are full-blooded Orcs, and our allies are Goblins if that makes any difference.

How viable is it to have a wall of wood (or pitch/tar/bitumen/oil/etc) out in front of your actual city wall, which you'd set on fire when you see the enemy? I was thinking this might buy a bit of time for our archers' volley fire to take out more of them, while denying their front line the opportunity to break in and do damage.

And we also thought it would be quicker than trying to build a wall that's meant to take a pounding, as this would basically be a pile (or pool) of flammables. Would I be correct in this statement?

We believe that they won't have near the archer count that we do, but we are going to confirm that with a scouting run this session. We are 100% positive that they won't have siege weapons.

You can set fire to your wall, but you can't compel the enemy to come anywhere near it while it's burning. If anything, orcs are not that dumb. They might be uneducated, illiterate barbarians, but they have enough int to know better than to run into a giant flaming wall of debris. And any orc that's gotten high enough in an army's chain of command to be...a commander in an army's chain of command, is probably going to look at that pile of stuff on fire and say 'hold, we'll watch it burn.' They have no reason to rush.

Now, if you could hide a bunch of flammables and light them after the orcs have passed, you could use it to try and trap them between your city walls and the firey wall, as a means of trapping them. But it sounds like the normal city walls are questionable about being held against the invading forces.

If this is a real siege with a real army (that just happens to be orcs) they might circle around the city outside of firing range and just wait until your city starves enough to give up.

I have to second the use of punji sticks, which is easier to set up. Dig a hole, sink some sharp sticks in there that are covered in feces, and voila: a spiked pit trap that also inflicts disease. You can then potentially use woven mats to hide the pits and stagger their locations around the city wall so that the only safe means of passage are on the main roads (where right and honest folk should be anyway).

It seems unlikely, but if you have access to a high enough level source of cleric spells and the material components for it, you could get forbiddance cast along the outside of the city walls (with gaps at the main gates). This spell deals damage to anyone who enters that isn't of the same alignment of the caster (and it's doubtful they would have any orcs of lawful good alignment). There's a will save for half damage, but even that would be 6d6/2 for orcs with lawful or good, or 12d6/2 for orcs that aren't lawful or good. That would kill an average orc outright, I think. The material components are super damn expensive though, so it might be better to key it to the ruler's alignment if possible, and cast it on just one location: the panic room area of the keep. It would at least make it very hard to take that one location with an infantry assault by orcs.

[RingofThorns]

Depending on the lay out of the city and the type of soil you could take all the displaced dirt from digging a trench for the spike and use it along with basic building supplies to make or reinforce bottle necks. Though if you dont think you can hold the walls, you could let them in and take the inverted castle approach