In relation to "dig deeper" - It's easier to simply drop a fortress on a hypothetical foe attacking from underground than it is to kill them all with a finicky

In relation to "dig deeper" - It's easier to simply drop a fortress on a hypothetical foe attacking from underground than it is to kill them all with a finicky army prone to wandering about near magma til they catch fire. For this reason I tend to dig out one fortress to drop and another fortress for living in. Survivors can of course be dealt with appropriately.