As far as I know (and my history is no better than amateur) the Celts (and the Gauls), as natives of Germany and France respectively, actually had quite a lot of rough, hilly terrain to fight on, and yes, the city of Alesia was located on top of a large hill. However, I think that's beside the point.
Remember that the game only uses absolutes like 'hills' or 'guerrilla' as generalizations; a tile with a hill isn't literally a big hill in a square, it's an area of terrain that could be considered hilly, whether it's many small hills, a few large ones, or anything in between, even (in the case of Alesia) one actual great big hill.
Likewise, the promotion 'Guerrilla' doesn't -literally- make you harder to kill when you're in hilly terrain; it represents guerrilla combat, which itself is irregulars or militia (civilian fighting forces) that make heavy use of surprise tactics, like ambushing and raiding. This form of warfare is ancient, and the Celts were -excellent- at it, as were the Gauls. (Most English/Scottish people can trace their lineage to Celts or Gauls who settled on the British Isles. The Scots also fared well in guerrilla combat against the English during the Scottish Wars of Independence, in the 14th century.) Think like the Vietnam War, but with less jungles and more highlands: high-momentum ambushes that charge through downhill forests; spearmen and archers hiding in trees while scouts watch from nearby cliffs, all at dramatically variable elevation; traps everywhere, deadfalls and avalanches, from rolling timber to rolling stones, forest fires, dammed rivers; local forces who don't do well in a pitched battle, but who excel at skirmishing, attacking the flanks or rear of a force and retreating to the cover of the terrain they know so well; never knowing how numerous your enemy is, or where he is waiting, or when he plans to attack, because intelligence is easy when your army is an army, but much more difficult when it's a civilian populace. Guerrilla warfare (there's a great Wikipedia article on it) was also really common for the Aztecs, even most Mesoamerican tribes throughout history really, which might also explain their bonuses in forests.
The point is, the defense advantage (to me) represents the local knowledge of the terrain that the native Celts (and Gauls) had over the Roman invaders, and the advantages they were able to bring to bear against them, and most of those advantages were in the form of surprise, as is the nature of guerrilla warfare. Again, you need only look to the Viet Cong to see how the ancient Celts would have defended their homeland from the Romans.
At least, that's what I figure.
By the way, historically, a dun was basically a well-defended settlement usually built into a hill or rocky area for added protection. If you've ever seen Braveheart, and seen those Scots living in their thatch huts built into rocky outcroppings and grassy hills, imagine a community of those surrounded by semi-natural walls of dirt and stone, several feet high. That's the sort of community most Celts/Gauls were raised in, so an intimate knowledge of the terrain was something that happened almost naturally, like the knowledge of how to use a computer comes to us. That's why the duns provide guerrilla expertise.
