Should probably add something in the rules about whether or not turning off features (invaders, cave-ins, the economy, etc.) is allowed. It's just vague enough now that someone might make an easy go of it.
So, how would I design it? Perhaps a meeting zone on a precarious bridge where all the pets meet up, above a magma pipe that provides central heating to the fortress halls through a series of well-placed tunnels and floodgates. And of course, a cooling system was also designed but someone forgot to install the grates for the individual rooms. Good thing they dammed up the main cistern, or the whole place would have flooded by now! And then there's the prisons, where many savage animals and siege victims are kept: strangely, that's in the center of the stockpile tower, which is the hub of the entire fortress. Of course, the most odd design decision of all was when they built a giant glass canopy over the surface level of the fortress to block out the hated sun, yet only supported it with a series of fragile glass pillars... The massive number of indoor spike traps are also a bit suspect, especially how they're built at random, but Urist McLeader insists that a good fortress defense requires unpredictable offense. And yet, nobody's ever seen a trigger to get those spikes to pop up.
Would be a terrible pity if anybody were to link all that up to a single lever. It'd be even worse if someone pulled that lever.