As it is right now, even enemy miners wouldnt be hard to defend against. Personally, I'm lava-flooding the yard. But I'm not doing it so much for defense, as I am to limit all the trash left behind. Trash that every dwarf rushes to haul away, instead of hauling all the more important things that need hauling. So I'm not really defending against the attacks, I'm defending against hundreds and thousands of extra haul jobs.
The way I see it, thats just a game tuning issue tho, and should be fixed eventually, so its temporary.
The big issue I see with invaders right now, is that they are all ground pounders. Aside from a couple odd monsters that sometimes join invasions, all the invasion troops are walkers, not swimmers or fliers. Adding swimmers would negate any water defenses. And adding more fliers would negate most lava and trap defenses. THEN things would really get hairy! Picture a hundred pegasi-mounted human invaders coming at you. How would you stop that? Steam might get some, but at least outside they could fly OVER the steam. Then what? Trapped halls could only get them when/if they land. All they would need, is some way to break thru doors, and you would have a bloody "house to house" fight inside your fortress.
Right now the game can be modded to add more fliers, but I dont know if there is a way to add fliers to invasion forces, or to get the troops to use them as mounts. But I dont think it would be hard to code that into the game. Then all that would be needed, is some way for invaders to beak down doors.
I think these things will get into the game in time.
But I think it should be limited to later invasion waves, like after year 7 or 8 maybe. That way invasions would get more dangerous over time, the way it should be in a game. That would still give new players a chance against the early waves, but give experienced players a challenge during the later waves.
I think we will be seeing this eventually. But it will likely take awhile. Toady cant just focus on this, and ignore the other things. Sieges are just one small part of the game. Its important, but so are other things.
Just my two coins worth.