So you went full on "Tucker's Kobold"?
Built as in built a mantlet, or formed, as in formed a shield wall?
0) The very fact that you have a story means that you recognize that this has already become a thing.
1) Does 5e have rules for shield walls and are they equally accessible to the PCs? In my experience shield walls even if you invent some rules to handle them only provide a major benefit when facing near peer opposition. If the rules you use give vastly inferior opposition a major benefit, they'll be absolutely game wrecking in the hands of the players turned against that opposition.
2) If built a mantlet or pavise that gave full cover, how long did that take or where they already on hand in the scenario? Did said tar, wood, and so forth already exist in the scenario or where they "plausibly" invented as a result of the problem the PC's posed? Is this such a common occurrence in the game world that armies have actually drilled in the deployment of tar covered bundles of wood?
3) In any event, how did the PC's respond? I don't see how anything you suggest remotely threatens a PC party. Using the one way viewing function of the spell they can easily see how at any enemy preparations while the enemy can't see what is being planned by the PC's. Flammables tend to be much more dangerous to low level NPC's than PC's, who have the option here of setting them off at the time and in the manner of their choosing The PC's have almost complete capacity to counter and disrupt the slow motion plan you describe.
4) Or, so long as they aren't surrounded, they could just ignore it. Huts don't collapse in 5e. You are thinking of earlier editions. Nothing in the spell provides for the collapse of the hut and any ruling providing such a collapse would go against the wording of the spell.
Shark, we usually agree on many things, but here I must disagree. First off, the players do not know, and generally will never know *exactly* how the Kobold tribe gains the wealth to store/stockpile/utilize all the oil, traps, poisons, and so on. Most don't, and are unlikely to find out the...
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