A goblin grinder sounds like a lot of work.
And I understand that dividing the main wave up makes it easier, but so does having more troops. I think that if I can get 50x1 hallway with 10x spear traps, every demon has to go through 500 spears before it gets to the military, which reduces their numbers to near-even levels.
And the reason I'm focused on military is that when I do get down into hell, I can't have everything be trapped: I need a way to manually kill the demons.
I didn't explain myself very well, and then I wanted to wait for a bit, because I wanted to make sure my idea works before I fully recommend it
(I'm working on a big project that's probably going to need just about every tile of usable space on the map, so I need the circus myself.)
Here's how I just took on the main wave with 6 pieces of blue, on a map with no other weapon-grade metal (yay nickel!), with zero casualties:
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dhbsb ->to the circus!
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d is door, h is artifact hatch cover (just there as bait, not over anything in particular), b is blue raising bridge with blue mechanism, s is a single blue spike with blue mechanism
I put a double hatch/pit thing (sort of an airlock pit) over the top, just in case I needed to safely drop bait animals, but I didn't have to use it all.
This way, they chase the miner out, but so long as the miner has even a few steps of a lead, the door closes and blocks line of sight, then the clowns focus on the artifact hatch-- from two tiles away, of course. Wait for them all to pile onto the square, block them in so they don't dodge, and pull on repeat. It seemed a lot easier to me than building 500 spikes in 50 traps.
If you don't have an artifact hatch cover, some other kinds of artifact furniture will probably work (all I know is hatch covers work, and statues don't.) If you don't have any appropriate artifact furniture, that's why I was talking about high speed switches-- you can probably still use a masterwork door or something, you just have to be sure to get your bridge up before they break down the door.
If you plan to wall off the circus all the way to the edge, I think you're going to have serious problems, no matter your military-- unless you somehow get a bunch of baddies in the center of the map, and manage to keep your masons from getting any aggro. When you're down to a few tiles to wall, every time the clowns get killed, they'll respawn (more or less immediately) at the only place they can spawn, forever and ever. So eventually, you're going to need a way to contain them, rather than kill them, so that you can stop the respawn. Same design as above makes for a great containment trap, although you might have to rebuild it at various locations, or else leave a trail of breadcrumbs (okay, a trail of chained cavies).
If you're fine with a mostly-colonized hell, you can avoid that problem-- so long as they've got one tile width to spawn at the map edge, with your wall the second tile in, they won't go crazy infinity-spawn on you.