Since some discussion has come up about the orc camp in my comments on my blog, here is some edited details of the route to the orc palisade in Felltower.
Session 119, Felltower 90 - Into the Orc Hole V
Once down the orc hole - essentially a hole in the dungeon floor going down a twisting, steep climb down, and then a cliff up to a cavern area - leads eventually to the "orc tunnel."
"It was about a mile in that Galen, scouting ahead, found a barricade across the 9" wide tunnel. Beyond it was a 6-7 yard circular cave [. . . ]
They kept going - eventually another two miles, hearing more and more howling and yowling and growls ahead as they kept going. Finally they found the exit - a tunnel mouth out to the bright sunlight of the day. Highlighted in the light were about a dozen devil wolves. The group decided Galen should shoot a couple and lure the rest into the tunnel where they could be killed. So they set up, and Galen shot and killed two. But the wolves retreated instead - the PCs couldn't determine why (they failed the relevant PER rolls I made for them to detect anything.)
The PCs advanced to see outside. They saw a wooden palisade, about 15-20' high, embedded in a solid stone embankment - clearly Earth to Stone was used if not Shape Stone. The palisade had many orcs on it, and two sturdy gates to the left and the right. Beyond them, separate from the palisade, were two covered towers with movable shutters and a tarp covering them - clearing artillery platforms. Orcs in the scores - maybe hundreds - were organizing and they heard owlbears and ogres, too. And goblins, of course."
So that's the palisade. It's a semi-circle of wooden wall embedded in a solid stone base - rounded and smooth, not angular and carved or laid down mortared stone. The semi-circle encloses the tunnel exit from the dungeon at a short distance - I didn't specify in the blog post but it's roughly 20-30 yards out at its furthest.
That doesn't leave any space for sheltered and sniping. It's completely cleared of cover of any kind. Any spot there is subject to direct and enfilade missile attacks. Getting over the wall means climbing 15-20' of wooden palisade embedded in a smooth-based stone embankment also 10' or so high. Or going through the gates, which aren't very advanced but still have towers to provide enfilading fire and plunging fire at anyone at the base.
The orc camp is largely obscured beyond it, but there are places from which orcs can see over the wall - towers on the far side of the valley, the covered towers within. It's a smart bet that something in those towers is aimed at dealing with foes coming from the dungeon.
So as much as siege equipment might sound useful . . . it's not clear how it would be trucked so far through dungeon corridors, down stairs, down more stairs, down a hole, up a wall, and miles across tunnels . . . and then assembled in plain short-range sight of the enemy. Sure, Galen can stand there with his Cornucopia quivers and shoot up the orcs, but math says eventually someone will critically hit a PC with Missile Shield and that Galen will critically fail and potentially break a bowstring. Meanwhile, the orcs have other non-missile options to annoy the PCs. It's not a non-starter for the PCs to stand around at the entrance and slug it out with the orcs at range, but it's not likely to be a decisive fight or a valuable one for them. It is a non-starter for them to essentially conduct a planned siege assault - they lack the ability to bring the tools to the site, the materials on site, and the skills to do so well. There is almost certainly a way to deal with the orcs violently, but the defenses make it a lot tricker than laying a partial siege from within a killing ground.
Why are the orcs so organized about dealing with threats from the dungeon? Is is only the PCs who keep massacring orcs within the dungeon? Or something else? It's not clear, and the PCs haven't really found out (and asking in town sure won't help.) But this is why it's not such an easy solution of applying tried-and-true anti-fortification technology to the orc wall conundrum.