As for castles in Poland, some of the smaller ones can be found with what seem like cellars too small to be much use but they link to caves under the castle. I've explored a few where the caves had access to springs and some where the caves were large enough to easily accommodate dozens of people.
One smaller castle in particular was built onto a steep hill and the bottom of the main tower led into a cavern that was high enough to be separated into two storeys. and the entry into the cavern was wide enough for a medium sized truck or APC to pass (although you would have to enlarge the ground floor entry into the tower, but in the T2k setting, I'm sure nobody is going to mind that extra work for the benefit of hiding their vehicles in a large cavern!)
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