Well, I designed the Wizard's Mansion to not only be an interesting and lively encounter, but also to be a didactic encounter; I wanted to force my PC's to start responding to threats. Because every single time they find something they don't understand the capabilities of, they end up running around like headless chickens and get wiped.
So, I'm endeavouring to make them do a few things; I want them to, once they realise they're under attack by the Razorfiend, fight it; but not just fight it, fight it smart.
When they, walking through the Marsh, encounter the Wizard's Mansion (they are aware of it, since they'd been sent into the Marsh to try to gain the Wizard's aid against the Red Hand), I described it as being very eccentric; it was painted bright blue and it looked, for lack of a better word, like it had melted slightly. The dimensions of the House didn't look right, and bits were bulging out; it didn't seem like the structure should be able to support itself. It was all twisted up and was covered in protrusions. When they walked up to the door, the doorknob opened its eyes and talked to them, telling them that "The Master is out, but he will surely be back soon". It was a pre-recorded message, as was all that the doorknob was able to say.
The inside of the house was likewise peculiar and esoteric. Unseen Servants, marked with blue bow-ties, floated through the air cleaning things. Two comfortable couches sat across the room from each other, one next to a Bookcase and the other next to a sea-chest. The bookcase couch also had two wooden stools next to it. Across one wall was a long table, with a stack of clean plates on one end, a box of cutlery in the middle and three empty tureens where food would be placed on the end. There was a glass with two flowers in it, also on the table. Finally, on the floor was a clearly visible Trapdoor. When opened, it was revealed that it opened to... floor.
Inside the sea-chest was an Assassin Vine. Its the sort of thing I'd expect a Wizard to keep in his chest of drawers.
The room was 10ft from floor to ceiling.
The second floor was a long corridor. You could enter a room to the left of the corridor which contained a very deep swimming pool. At the far end of the corridor was a ladder, leading upwards.
If you were to jump into the pool and swim down, you would realise that the dimensions of the pool were impossible compared to the dimensions of this floor; it goes down 30ft. You can easily push through the bottom of the pool... which allows you to pop up where the Trapdoor is on the first floor. There is also a highly visible hole in the floor of the corridor, which would allow someone to jump down to a spot in front of the wooden stools back on floor one.
This floor was 30ft from floor to ceiling.
The third floor was where the Wizard's bedroom was, and a separate area contained a Gelatinous Cube held captive in a glass container (it had a few hamster skeletons floating inside of it). Next to it was a black space that would magically remove any material tossed into it.
Like the previous floor, there was a hole that a character could jump down, which would allow them to get into the pool.
This floor was 60ft from floor to ceiling.
Finally, there was an open-topped study at the top of the tower that could only be accessed by creatures with the capacity to fly, since the house-side entrance required a character to get up 60ft with no handholds, and the outside side entrance is 120ft up. The effects of the weather were staved off by a magical barrier which covered the top of the tower.
I designed the House with these four levels to set up a fairly simple principle; each level you go up, makes the fight against the Razorfiend easier.
The Ground Floor was fairly boxy, and the whole place is really clumped and covered in furniture, obscuring charge vectors. It also has two entrances that weren't easily guarded against; a hole in the roof that the Razorfiend can attack down from (which it did to open the combat, insta-gibbing a friendly NPC) and the one-way trapdoor that you can magically get to from the pool. Furthermore, The Razorfiend is fast enough and can jump well enough to dart down from the stairs, hit someone close by, and the retreat back up the stairs.
Staying down there puts the party in a killing box. They can't get out through the door; it has locked itself, telling the party that all doors are locked past sundown. Bashing the door down is all but impossible, it being Magically Treated Reinforced Iron Walls (Break DC60, HP360, Hardness 20, Energy Resistance (All) 30), so they're forced to stay in the Wizard's pad and deal with their problem.
If they go up a floor, they suddenly can't be attacked unaware as easily. It can't get at them from the roof (too far away) and it can't attack from the trapdoor.
They go up to floor three? The Razorfiend is going to get ruined. It can't attack from the walls or ceiling, it doesn't have the AC to engage in a protracted fight. They get it up there, and keep it up there, and the players win.
Guess how many floors my party went up? Not a single one.
Details about how my Party Handled the Wizard House
Spoiler
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They stayed down on the ground floor, and hoped that things would fix themselves, and then as a result had to fight a Gelatinous Cube, the Greenspawn Razorfiend and that Assassin Vine.
The Razorfiend's tactics weren't that advanced; it made its first attack from the hole in the wall above the stools, then Breath Weapon'd them from the trapdoor. So, I'd shown them the two ways that the Razorfiend could access the room without using the staircase. They sat there, readied actions to attack it if it came close, and waited. Only one player, who had actually taken the time to investigate the house earlier, did anything proactive. He crouched on the stair-dog and looked down the corridor. But, since that's right next to one of the doors to the pool, he didn't have time to react when the Razorfiend opened the door and gibbed him. 15-20 Crits on a Wingblade are terrifying.
The rest of the party knew they couldn't get out, and after hitting the door with some concentrated fire and being warned that the house will retaliate if they continue, they decide that the best plan is... to call out to the Wizard and ask him to call off his monster.
So, the Razorfiend is clever, packing 12 Int. It has seen, at this point, the Party slaughter the Assassin Vine with the help of Dragonfire Inspiration. So, it doesn't want to deal with that, and as such, goes upstairs to retrieve the Cube Jar. The party makes listen checks, but none of them can hear the scraping on the floor above them as the Jar is dragged across the floor. So, none of them have any warning when the Trapdoor is kicked open and the Gelatinous Cube Jar flies up out of it. The Trapdoor falls closed... and the Jar smashes on it. The Cube expands outwards, and Engulfs the Party Bard and Wizard in its surprise round. Both of them fail against its Paralysis, and start to melt in the acid.
The rest of the Party kill the cube in short order, but with their two remaining spellcasters downed all they do is just sit around waiting for something to happen. So, the Razorfiend waits for the DFI to wear off, and then darts in, executing both Wizard and Bard in short order. At this point, two of the three remaining characters are sitting pretty in the single digit hitpoints, so it stays where it is, confident it can take them.
All of the Melee Fighters who are still up run in and try to beat the Razorfiend to death. The first damage they do to the bloody thing, and they manage to knock it down to half HP. It retaliates, dropping one character to -6 with a Wingblade, missing with the second and scoring a Crit with its Bite.
I offer the remaining party members a compromise; I'll let them decide who takes the Critical Bite. The downed Swift Hunter, or the Anti-Paladin still on Full HP. In true Anti-Paladin fashion, he refuses to take the hit, so the Swift Hunter is eviscerated.
Next round, the Anti-Paladin hits the Razorfiend with a devastating full attack and drops it to -1. They then cut the Razorfiend's body to ribbons, just to make sure its really, definitely dead. Lanikar's body (and some of his Owl's) erupt out of the digestive tract and we rap up the session.