Wizard Towers

Wizard towers are tall, slender buildings that are home to a wizard. They may be magical in nature. They may be located in rural areas, on magical ley lines or other magical places, or within towns, cities, or castles. They may have a hidden entrance or an entrance protected by magical means. They may contain portals, libraries, magic kitchens, and other rooms. They may be decorated with runes, gargoyles, and magic symbols. They may contain treasure, spell books, and magical items. They are home to wizards, witches, elementals, djinn, and magical constructs.
When designing wizard towers, consider what aspects of this structure may be magical. Are there rooms that defy spatial constraints? Are there magical traps in place to protect against intruders?

What is this tower used for and is this type of building even necessary?