Enough lava... back to the Toadwallow Caverns. Here's the full revealed map, without minis. In addition to presenting the cave layout, I'll provide analysis of what I think is a remarkable example of 4e encounter building.
That's it - not too flashy, but it has one very interesting setpiece - a gargantuan dragon skull in the center of the cavern. To sum up the cavern's purpose and backstory, it's inhabited by chief Gloorpk's bullywug tribe. The Iron Circle bad guys have hired Gloorpk to harass the steading of Tor's Hold, so they're too busy to aid any resistance effort made by the other occupied villages of Harkenwold.
Enter the do-gooder adventurers, looking for work. A local druid suggests they can help to fight the Iron Circle by taking out the bullywugs of Toadwallow Cavern, thereby freeing up the fighters of Tor's Hold to aid in the resistance.
Toadwallow Cavern is a nice small dungeon romp. In the module, they've cleverly designed three unique encounters within the same space. For us Dwarven Forge enthusiasts, that means you get at least a night or two out of a single setup, and it's unlikely you'll have to build something else for that night's game. And the encounters are unique and memorable, each with their own challenges.
The first encounter starts when the party shows up at the cave mouth. It's ten feet up, and the mouth is a waterfall. There are four beefy, non-minion bullywugs defending it, along with three stirges in a nearby pool. The short cliff plays an interesting part in the battle. The PCs have to climb it, while the bullywug sentries prepare to ambush them if they aren't stealthy enough. In our battle, the first one was not stealthy, and was rushed by all four bullywugs. The battle was brutal, and the controller-ranger managed to use the drop to level the playing field.
In this first encounter, the layout cleverly restricts the battle to just the cave mouth. There are more enemies within, who have a chance to prepare while they wait to see the outcome of the first battle. See how the sightlines are restricted to just the entry cavern, concealing the dragon skull beyond (purple lines). As long as the sentries contain their space (green semicircle), the party must deal with them before moving further in. The fact that the second encounter won't trigger until they move inside the cave gives them the option to take a short rest before moving on.
The second encounter is against chief Gloorpk, a spellcaster and his toadie minions. A couple of gray oozes lurk in the pit, and will assist when the battle begins. Not only does the skull make for a nice memorable set piece, it is hollow, and can fit a medium creature inside. From under the skull, the creature has superior cover (+5 to all defenses), and can trace line of sight and effect from any part of the skull's space. Gloorpk starts inside - very hard to hit due to his already high defenses. The party is swarmed by minions while Gloorpk uses potent ranged magic from under the skull's protection.
It was great. The party had to decide whether to try lifting the skull to get him out (provoking op attacks in so doing), to destroy it (I gave it 400 hp, AC 1 and provided only cover at 200 hp), or to try pulling him out. It was a great opportunity for creative tactical play. They ended up sliding him out with an effect that required him to make a saving throw, and finished him off after that.
That's the end of the second encounter - the big bad is defeated! But there's one more encounter. While the PCs are searching the cave, they hear a loud croak from outside the cave. Gloorpk's lieutenant and a war party are returning with a captured halfling boy, and they see dead bullywug bodies outside the cave. The party now must defend the cave from the war party, for a cool role-reversal encounter. That's tomorrow night's game, so we'll see how it plays out. They have already dragged the dragon skull to the front of the cavern, and they'll have their ranger inside pelting the bullywugs as they advance.
I like that the second encounter seems like the last, with a surprise third encounter. It's also unavoidable, since they are pinned inside the cave. After defeating the war party, the PCs are faced with a new option - return to the quest-giver druid or return the halfling boy to his family in another area, sending the story in potentially different directions.