As the others said, moisture definitely, but damp or rotten wood even more so. We live in an old cottage, and we're always hoovering them up from round the front and back doors, and in one particular bit of skirting that has a tiny gap under it and into the old stone wall. Even more so after rain. A few of my friends have similar old houses, and similar invasion in the wooden bits.
A week or so ago, my son found the ugliest spider I have ever seen. I looked it up on the net, and it was a "Woodlouse Spider." Never heard of it, but it eats lots of them apparently, so we let it stay!!