One thing I've noticed is that invading armies (endless migrants included!) will quickly and easily cross what looks like the roughest, most inhospitable, exhausting, dangerous terrain, and emerge from it fresh as a daisy, and often able to walk all over a well rested, fully alert and equipped, defending army.
As an experienced hiker, I can tell you that marching up and down steep, unpaved terrain, often ankle+ deep in sand, mud, snow, or even loose, razor sharp stone, which has a decent chance of being covered in thick foliage, and infested with all manner of bugs, and sometimes even rattlesnakes, is not easy in the least.
Add to that the numbing cold and slippery ice of winter, the ubiquitous mud of spring, the heat and sweat and snakes of summer, and even the piles of fallen leaves and general dreariness of autumn.
Terrain bonuses and penalties deserve a place in a game as deep and complex as DF, and would add enormously to the potential for Fortress defense strategy.
It makes sense that we should be able to incorporate natural conditions into our Fortress defense, and that invaders should be granted every opportunity to experience the following:
sore feet, including blisters, sprains, and even cuts (are they wearing shoes? if so, are the shoes good quality?), as well as skinned knees, hands, and elbows from even short falls, which may lead to infection,
sunburn and heatstroke, especially in heavy armour, or skin freezing to metal,
all the effects of rain, lightning, snow, hail, high winds, etc.,
uncomfortable dirt and grit getting into poorer quality armour, and iron and steel armour rusting,
various combat penalties from sweat, and the greater risk of illness in general, from poor hygiene, since finding places to bathe may not be an easy or safe task,
thirst, hunger, and general fatigue, and the difficulty finding pure water and fresh wholesome food, as well as a safe place to sleep, and food and water themselves becoming spoiled,
bad falls (up to and including off of steep cliffs),
insect bites--which may carry poison and/or disease, and occasional animal attacks,
thorns and poisonous plants, including poison ivy,
numb fingers, chapped lips, and frostbite, and the difficulty-not to mention the potential risks in enemy territory-of building a fire and shelter, and becoming fireblind (temporarily losing night vision),
becoming (literally) bogged down in mires, quicksand, along shores, mud flats, or the floors of many caves,
additional mud, landslides, or even avalanches, caused directly by the marchers themselves,
And mounts, now that they're becoming a significant factor in ambushes and seiges alike, can also suffer falls, cracked, split, or rotten hooves, throw shoes (if they even have shoes), sprain or break legs, suffer panic attacks from loud noises or insect bites, fatigue and exhaustion from overwork and/or overburden (which can lead to their death), and even increase the chance for attacks by very large predators, such as rocs or dragons.
Most of these conditions wouldn't be experienced by the defenders of a prepaired and well-stocked Fortress for atleast a month or so into a seige, and to all of this, you can add myriad traps, pitfalls, moats, walls, and other defensive works, drawbridges, ambush sites, guard animals, poisoned or simply removed food and water sources, interesting catapult loads such as: bee's nests, animal corpses, quicklime, and red hot sand, aided by intentional deforestation and removal of plants, or even salting the soil outside your Fortress, to create clear lines of fire, fields of caltrops, broken glass, poison ivy/oak/sumac, oil slicks, punji sticks, gazebos, hideously terrifying and dangerous koi ponds, etc.