much maligned is the spiked ball. It also has 3 attacks and copper spiked balls are just as effective as copper discs, silver spiked balls regularly cause heads to crack even through helms.
2 iron serrated discs and 2 spiked silver balls in a trap, making it a field of 5 deep will kill a lot, and what passes through will be seriously injured.
Personally, after this field I have 3 rows of upright spear traps with copper or glass spikes, then a row of cages and again 2 rows of upright spears. After that I tend to make a pattern alternating cages, spikes and weapon traps. A 20 tile long corridor in this setup will kill anything. Most likely, you wonder about the spikes.
I assume you use some cages too, to get sparring dummies and leaders have a habit of getting trapped, making the rest of the squad hang around, most likely jamming a lot of weapon traps. The next squad runs right over it. By setting some cages (I actually start with some spikes at the entrance, then cages spikes alternate untill I get to weapon traps way in the end) with spikes around, you trap 2, 3 or more, then kill the squad with the spikes. Next squad gets to the next row, gets trapped and killed. Now all cages are full, spikes on repeat and anything that gets past the spikes gets chopped and smashed by the spiked ball/serrated disc combination. If anything gets through, my militia (non-danger room, all natural) can probably deal with it (2 marksdwarf squads, 2 mixed with axes, hammers and swords, 1 lasher squad, 1 recruits/wrestlers squad).
It was in fact so effective that I made 2 new entrances to the fort with different traps, fewer traps or whatever, because nothing was coming through
The good thing about the spikes is that they also affect goblin demonic leaders and titans