There are many traps that get overlooked or have secondary benefits. A lot of people use Brimstone almost exclusively but I feel like the Floor Scorcher is a better trap. It has physics effects in addition to its AoE damage, a fast reset and adds to combo multipliers in a big way. You can put one beside a cliff for example, pointing inward toward the lane and it will flip monsters over the edge in addition to doing damage to things.
The steam trap is awesome too, its only like 250g and you can put a grinder or wall blade 2-3 tiles then it will flip things upward into those traps which adds greatly to your combo scores. Haymakers work well on ceilings for flyers in addition to owning stuff on the ground. Using spring traps to fling things across gaps into void walls is comedy gold too.
Anyways I highly encourage people to experiment and dick around. There are a ton of viable trap scenarios for every map, I actually respec'd between every single Nightmare level to keep trying new things.
Also, AutoBallistas are really the only good trap against flyers unless you can place arrow traps on the path the flyers will take.
I don't know, I hate them because they don't prioritize air targets like archers do. So if your ground box is anywhere near the ballistas, they will waste their cooldowns on kobolds and shit while the flyers go straight to the rift. In the story levels this isn't a big deal but in Endless mode there is no good solution to flyers. Shock zappers suck and archers are wasted money. You basically just have to handle them yourself.
Playing: Dark Souls 2: Scholars of the First Sin Watching: Mr. Robot