There are lots of reasons for forts to become ruins. Perhaps they were abandoned after a war, or overrun by brigands. Why exactly it happened might not matter, but adventurers definitely like exploring them. Whether for loot, a land claim, or because someone or something else is there that disagrees with their sensibilities. Here are some system agnostic items (but still aimed in the flavor direction of Beyond the Veil) you can include if you are so inclined.
1D12 Items in a Fort or Ruin
1. Captain's Mace - This flange mace is masterfully made (+1) and also functions as a parchment carrier. The handle can be opened to store a map or a handful of letters. The pommel doubles as a wax seal. It comes with a worn brass belt ring. It probably has something stored in it.
2. Weatherproof Fuse - Caught in a light drizzle and unable to use their rifle, a musketeer's effectiveness is reduced significantly. Someone apparently figured out a fix to this vexing issue. This fuse can be affixed to a matchlock rifle, pistol, or used on its own. Once lit, it functions for 2 hours per foot, regardless of wind or rain. It is extinguished if completely smothered or submerged. The coil is 2d2 feet long and has a leather carry pouch stamped with "Property of N. Daviess."
3. Watchman's Horn - A short, carved, ivory horn with holes drilled into it in much the same manner as an ocarina. The mouthpiece is made of copper. It can be heard clearly up to a mile away over an open field. Someone has carved symbols next to the holes with distinct meaning. Fingerings perhaps?
4. Liar's Dice - These dice always roll slightly higher than normal when the owner tells an obvious lie when the roll is tossed. Add 1 to the final result of a check if a lie is told while using them. The dice are not obviously enchanted.
5. An Incomplete Ledger - Mostly boring bureaucratic and logistical scribblings. The last page is incomplete and smeared with ink. After investigating it more closely (an easy but mind-numbing task), it is discovered that there is a hidden storeroom in the complex with a level-appropriate amount of cold, hard coin, expensive liquor, and a non-magical but heirloom quality sword of the now deceased commander.
6. A Trusty Cloak - This cloak is made of canvas and lined with soft hare fur. Lead is sewn into the hem and as such it does not flap in even strong winds. It weighs three times as much as a normal cloak made of the same materials (minus the lead).
7. Rope Retractor - An ingenious makeshift device that when affixed to a rope, the user can turn a crank to tightly coil any type of cordage up to 100 feet long and up to the thickness of ship's rope. This process takes 1 minute, after which the rope coil can be removed from the device.
8. Ration Crate - Open, but suspiciously untouched. Six rations wrapped in tattered cloth. Each ration is hard as a brick, smells strongly of vinegar and salt, and seemingly inedible. Not even maggots dare touch them. If soaked in water for a half-day, they become technically chewable and each one can sustain an adult human for three days, or someone smaller for six. No one will enjoy it and a difficult test of one's willpower is required to force it down their gullet.
9. The Log of a Former Adventurer - Not only is there a complete map of all the unlocked or accessible rooms of this ruin, but the location of another ruin notated as "Hells below, never seen something like that before! Danger!" next to a poorly drawn basilisk. There is also the amply provisioned skeleton of a long-dead adventurer, killed via an unfortunate accident.
10. Unlabeled Alchemicals - A milkcrate of 1d8+1 crystal and wooden flasks, all the labels long since worn away. Use your favorite potion or poison generation method to populate the crate. One of the potions is always broken. Its flask has a weathered note tied to the remnants of the neck that reads "Caution: Do not ingest! Contains drowning mildew!"
11. Unusual Bedroll - Originally mistaken as a rather thick coin purse, this lightweight bedroll folds down to fit in the palm of your hand, or fit in your pocket. Folding the bedroll takes 10 minutes and unfolding takes 1 minute. It is made of a material you are unfamiliar with and smells faintly of dry blood and ozone.
12. Onyx Monocle - Ornately decorated with bronze fixtures, there is a necklace locked in this box. It is a silver chain attached to a paper-thin red onyx intaglio. When held up to the left eye, one can see magical energies through the onyx sheet.