[luguvalium]

Here is another idea: It may not be easily turned into a solo adventure, but consider it a challenge. You are in charge of a stronghold in the wilderness that frequently gets raided. You need to carefully consider how to allocate your resources: Low level fighters, high level fighers, wizards, stronger doors, traps, etc. Once you've placed you resources, randomly generated raiders attack and see how far they get. It could be a human or dwarven stronghold, or for a twist, an orc stronghold that gets raided by humans.