Sometimes the randomness of how things attack can add a lot as well, especially with the graphical simplicity.
By this I mean I remember once I had a fortress that was basically at war with the unicorns living outside it (this was way back, in the 2D version, by the way).
Things were going ok right up until I sent my woodcutter across the river to cut the tree's there, and I think he must have gone too close to some unicorns or something because they all just instantly stampeded across the bridge and right past the entrance to my fort. I still had a lot of my fort outside at this point as well, so I lost a dwarf, had a dwarf very badly injured and got a bunch of dogs killed. I imagined that it was basically the unicorns considered the land across the river to be theirs, and so my woodcutting would practically be an 'invasion'.
After that stampede happened I quickly moved all my workshops inside, even though it meant quickly mining out a few area's and permanently messing up my entrance layout.
Another interesting incident from that fortress was when I got my first immigrant hunter. I decided that it was time to get revenge on the unicorns, and sent him out with his crossbow to hunt some down.
When he got to the first group, he had time to fire off a few arrows, killing one of the unicorns, but then the rest got to him. Strangely, they didn't kill him. Instead, they messed up one of his legs (red-wounded) and poked out both of his eyes. Then they left him alone and walked away. I'm not sure what the actual reason for them walking away was (do animals stop fighting when you go unconscious? ) but I imagine that they decided that as punishment they would simply make it so he was permanently bed-bound and couldn't hunt anymore.
He ended up starving to death when I gave up and sent all my dwarves to wrestle with the unicorns... I forgot that he was there until my dwarves were all either dead or dieing...