Base of Operations is one of the original Dungeons & Dragons adventures released in PDF format on the Wizards of the Coast website as part of a quasi-monthly series. The adventure involves clearing a keep of a band of monsters, and possibly becoming the new lord or lady of the manor. Intended for four 5th-level characters, and uses the version 3.0 rules.

From the website:

Free for the Taking: One Keep...*
*Must first evict current tenants

A group of vile humanoids has taken over an old, ruined keep within a day's walk of the nearest town. Local leaders greet this news with alarm, for they don't wish to see a stronghold for evil humanoids so close to their settlement. So they offer a deal to your group of adventurers: Root out the humanoids and you can have the keep as your own base of operations.

D&D Ed Stark presents Base of Operations, an original Dungeons & Dragons adventure suitable for a party of four 5th-level player characters. If your group is in the mood for a full-on assault, this adventure dishes up a few nasty battles, some tricky tactical maneuvers, and a chance to take charge of a specific area of the campaign world.