This literally just happened, and I'm reeling from the realization.
I started a little fort on a prime location: a junction of two streams into a minor river, with a volcano just to the side, and sand everywhere. Savanna biome, with elephants and leopards for icing. I'm building a megaproject right over the river junction (special trap tower I've designed and want to see how it works; draws water up from the river and then dumps it out the sides to wash any invading force right off like a bad case of fleas) and have just a little hole in the side of the volcano to house my 80 dwarves. No defenses whatsoever, just some cage traps set to catch elephants to start a meat industry.
Spring of the third year, the outer wall and ramps of the tower are 2/3rds complete (Using SPEED:0 because I can't be bothered to wait and deal with the inevitable tantrum spirals from the dwarves living in a muddy little hovel for 10 years while their fort is built brick by green glass brick; did I also mention I am not digging below z-1 by choice?) when the first titan appears:
The Forest Titan Senama has come! A gigantic quadruped composed of rutile. It has two long, hanging tails and it has an austere look about it. Beware its webs!
Sonovabitch. Quick burrow designated, everyone inside, replace the double doors with a 2x2 raised bridge. Manage to get everyone in and the bridge up while the titan chases around a few stray animals. Upon getting in and seeing the titan kill the dogs and such, I decide to bring the fight to it. Seven dwarves chosen at random become a rag-tag militia force. They find weapons and armor where they can (I haven't made anything except for some elephant bone-and-leather bits) and charge the beast, some with no weapon at all. The militia commander, a retired hunter (retired because he wouldn't stop killing elephants while I was trying to catch them) is still looking for a weapon and isn't with the charge.
The other soldiers hold the beast for a time, with the three armed dwarves ripping up every part of its stony body, but to no avail. Sword, mace and spear each fall as the dwarves are destroyed one by one, and the titan stands above the corpses a moment, screaming its victory. It begins to head for the vulnerable fortress.
But lo! The commander has found a bismuth bronze axe, and seeing his fallen comrades behind it lets loose a great bellow and hurls himself at the beast, his axe swinging madly! One cut removes a tail, another its foot! The titan cannot keep up with the onslaught of this lone dwarf, and is forced to the defensive! Finally, with a grimace of hatred bordering on insanity, the commander drives his axe through the beast's midsection, and its halves slump to the ground with a sound of grinding stone, the life gone from the monstrosity.
Exhausted from his victory, the commander hefts his axe and starts back toward the camp, where a new party is picking up steam, no doubt to honor the brave warrior and his fallen friends. Yet just as he sets foot on the threshold, a movement in the distance catches his eye. He looks to the hill in which their home is carved, and green skin and iron coverings cause the hairs on his neck to prickle.
A vile force of darkness has arrived!
Gritting his teeth and hefting his axe, he turns away from the partygoers and tells them to keep a barrel of ale ready for his return. There is hunting yet to be done, he declares, and sets off to meet the charge.