Yeah, you really have to haul ass with the defenses if you don't want to get shredded. I'd make the Spawn arrive later if I didn't find the mad dash to get an effective military set up so perversely enjoyable.
I think I should also mention that Spawn are immune to death by magma. They do burn, but that just makes them more deadly.
EDIT: Also, the Spawn's "Special Surprise" only works on vanilla dwarves. Any other species *cough*Maej*cough* will still be ripped to shreds, but only dwarves get the Special Surprise. Modded dwarves might still be Surprised so long as you don't add or remove any castes.
- Spawn do not bleed nor breathe and their brains don't do anything. They only die when their hearts are destroyed, they are chopped in half, or (due to an oversight) they are beheaded.
- Spawn don't feel pain.
- Due to their lack of bleeding and obsidian hearts, immersion in magma does next to nothing to Spawn. I think it does slow them down due to their legs being injured, but it has the nasty side-effect of lighting them on fire virtually-forever (I'm considering making them flat-out fireproof, as there really isn't any way to combat that when it happens).
- Spawn have TRAPAVOID and LOCKPICKER (BUILDINGDESTROYER led to pathing errors, sadly), so most passive defense methods don't work.
- Spawn have no weapons, but they can bite through armour with frightening efficiency due to natural skill and force multipliers.
- Spawn begin to attack roughly at the same time as goblins.
If a Spawn successfully pierces into flesh when biting a dwarf, the dwarf becomes infected with a syndrome that may result in them permanently turning into a crazed Spawn a few months later. Dwarves have a roughly 25% chance of turning, though an odd save/load bug means that saving and reloading between a dwarf being bitten and the point where the dwarf possibly turns guarantees that they will go Spawn. Note that the initial bite has to break the skin to infect dwarves; further damage from latching on after biting does not pass the syndrome.