A den is the place where your family kicks off their shoes and unloads after a long day. It's not going to be the most posh space in your house, and it will definitely be privy to more abuse than your formal dining room. However, it is important to not neglect this room when decorating, as it needs to be the place that you and loved ones will want to actually spend their free time.
Because of this, a den should feel warm, informal and inviting, but also have an aesthetic that is aware of the activities, or lack thereof, that go on in the room. Just like the furniture that goes into this room, durable window treatments need to be considered when choosing your decor.
One fabric choice that works great in this kind of environment is damask. The design can come in velvet, cotton, silk or satin. In your den, you'd want to go with cotton, as it is the least formal and the most durable.
Damask patterns are a good choice for the den because they feature visually complex designs that showcase the fabric's texture instead of making the color palette of the window treatment the key feature. Often the hues used in a damask curtain are very similar to each other so that the pattern evokes a contrast of sheen rather than of colors. This approach makes it so that the pattern avoids being loud or offensive, but instead more neutral while not being boring.
In rooms like dens, where the color scheme tends to be in the center of the spectrum as opposed to employing extreme or experimental hues, a damask curtain will stand out while not ruining the look of the room.