An adventure for character levels 3-5

Remember the good old days, when adventures were underground, NPCs were there to be killed, and the finale of every dungeon was the dragon on the 20th level? Those days are back. Dungeon Crawl Classics don't waste your time with long-winded speeches, weird campaign settings, or NPCs who aren't meant to be killed. Each adventure is 100% good, solid dungeon crawl, with the monsters you know, the traps you fear, and the secret doors you know are there somewhere.

In this all-new adventure for levels 3-5, the characters come across an ancient wizard's tower. The nearby keep has been reduced to rubble over the ages, but the tower is in perfect condition. It is surrounded by an impenetrable force field that cannot be breached - not even by the ghost of the long-dead wizard, who has been trapped within for centuries. Surely there must be great treasure within this magical abode. But how to get to it?