Quite often in a big old fortress, there are places no-one's been for decades. Sometimes it's an ore vein, a mausoleum or a dead dwarf's house. Hell, sometimes it's even a whole walled-off network of rooms.
Now, if you leave a room alone for a few years, it doesn't stay pristine. Dust gathers, moulds and fungi spread, worms eat wood, moths eat fabric, and spiders spin webs upon webs upon webs. Old fittings will eventually crumble and crack and tarnish.
For things outside, roofs should leak, mosses and lichens should flourish, perhaps even small trees. Eventually, there might be nothing but a stony shell of a once-fine mansion, perhaps only a heap of worn masonry.
My basic points are:
Add dust. Coatings of dust should eventually gather on things, slowly vanishing when trampled by dwarves or cleaned up by a cleaner.
Allow the generation of woodworm and spiders and such in long-empty chambers, perhaps damaging wooden furniture and making accretions of old webs into an almost solid cloth.
Damp and Mould should spread on furniture or even ancient food.
If left long enough, fittings might simply collapse into dust, depending on quality and the vermin in the room
Outside roofs should eventually leak, mosses and lichens should spread like very slow grass.
Very old and long abandoned buildings should crumble. We're talking several decades or even a few centuries here.