One characteristic of more modern times is that of a more city based population. This lends itself to terrorism both because you have more available civilian targets in a smaller geographical area (this also helps the "terror" to spread after you've committed your act) and provides relative anonimity to people who wish to infiltrate that community.
But if I was a medieval terrorist (or one in a typical fantasy setting), I'd probably not strike directly at the cities at all. If I had a group of fanatics that I could train to any reasonable degree as combatants and outfit with armor and weapons, I'd think (though I might be wrong) that I could bring a small kingdom to its knees.
Let's say that I can outfit a dozen men with armor and weapons (not fancy ones, let's say Scale Mail, Spear and Shield), horses and train them to some minimal degree of martial proficency (Warrior 1's). I then move to the outskirts of a small farming community. At nightfall, I break my group into three groups of 4 and each heads for a farmstead where they kill the family and farmhands there, loot the place and steal any other horses. Any other livestock are killed and their bodies thrown down the well. Then they move on to the next farmstead.
By morning we've probably killed most if not all of the local farmers and at that point we set fire to the fields. Then we ride away (possibly long and fast enough to kill the horses we stole last night, but hey, they were free) and rest up for a couple of days. Then we hit another farming community in a distant part of the kingdom. Rinse, repeat.
I'm guessing that after the 3rd or 4th iteration of this exercise, the rural farmers of the kingdom will be terrified and will start to move their families into local forts or fortified towns. The King will no doubt have his "top men" out there trying to find and kill me, but it may be too late already. If I've chosen my timing right, nearly an entire year's worth of crops may be burned or rotting in the field with nobody to harvest them. Even if they kill me and my men, it will take considerable time and effort for the farmers to clear out their wells and rebuild barns and houses.
The kingdom may well be economically crippled by virtue of the ensuing famine or the large scale import of food to avoid one. Regardless, my political point will have been made and even if I'm killed, others that serve my cause can repeat this process the following year with less than 1,000 GP of startup cash.
By golly, I think I've just come up with the seed for my next campaign!