Is there a functional reason for wizards to live in towers so often?

Question

The concept of wizard towers seems to predate D&D, but has there been a functional reason for this added to D&D? I'm going to be using the answer specifically in 5e, but wizard towers transcend versions of the game so I don't believe this question is specific to 5e.

Is there a functional reason for wizards to use towers so often? Or is it just a cultural/traditional thing wizards do?
For clarification: Is there a first-party reference that explains why wizards often use towers?

For example: in one of the most popular 5e modules, Lost Mines of Phandelver, there is a wizard's tower in Thundertree.

I've found this old popular Reddit thread which mostly has some joke responses with a few references to potential historical sources of the concept of wizard towers, but nothing notable that is "functional" in the sense of aiding a wizard practically.

In an episode of Critical Role Campaign 2, the DM referenced the fact that a good number of spells require arcane circles in the ground and that this is why wizards lean towards towers. This is cool, but I can't find any references to this being the why and it doesn't really explain the need for any sort of height.

Some possible answers, if they were supported by sources, might be: they're particularly easy for wizards to create; they're particularly good for wizards to research magic; or the height advantage is a great complement to some class feature.

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This question is not looking for speculation or historical reasons. Answers should directly quote a D&D book that gives a clear direct, or indirect, benefit to wizards for living in a tower.

Answer 1

The Player's Companion gives specific benefits for a Tower

The Cook and Marsh Expert Rules for D&D were released in 1981. They were part of 'Basic D&D' when what was then-called 'AD&D' was in the latter part of its first edition. As such, they certainly did not establish the within-game trope of wizards building towers, but they played into the trope that already existed, both within-game and within fantasy literature before the game itself. Since they detailed levels 4-14, they covered what happened at 'name level', when characters could become landowners. While demi-humans built 'strongholds', fighters and clerics built 'castles', and thieves built 'hideouts', magic-users specifically built 'towers' (p. X7). At the time, the only mechanical benefit of this was that

1. constructing a tower attracted a number of low-level apprentices to serve the magic-user

Later (1983) Frank Metzner revised the Expert Rules and in 1984 authored the two books of the D&D Companion Set. Here there was a more deliberate attempt to introduce multiple benefits for wizards with towers as sort of a post hoc justification for the motif. Metzner was once an active member of this site and could certainly provide more background, but he hasn't been 'seen' here for over a year.

As the Companion Set detailed levels 15-25, much of the information given was for high-level characters creating and administering Dominions. For the magic-user class, the possible non-adventuring options were to either build a Tower (now capital-T), or (for a magic-user who "cannot or does not want to maintain a Tower") to take service as a court wizard for an existing ruler (and see Player's Companion pp. 8 and 19).

For a magic-user who constructs a Tower, three specific benefits accrue, which can be considered class features 'unlocked' by Tower construction:

2. The Tower serves as the character's Stronghold for the purposes of Dominion rule (the term stronghold now applied to any fortified structure from which a character ruled, not just for demi-humans).
3. Once the Tower is built, a dungeon may be constructed beneath it. The dungeon will attract monsters, whose treasures and body parts may occasionally be harvested.
4. The Tower will also attract wandering magic-users, who may then be invited to help with magic-item creation, as per the revised Expert rules.

All of the revised "BECMI" books were later collected in the Rules Cyclopedia. There, the rules for magic-user Towers are repeated (pp. 20, 134), and additional benefits added:

5. If a Tower is built on the lands of a ruler, the ruler will issue a proclamation that their subjects are to avoid bothering the magic-user, and
6. If the Tower is attacked, the ruler will come to the magic-user's aid (unless the attacker is another magic-user).

Answer 2

There is no functional reason, it's a trope

You have tagged your question specifically for dnd-5e and dungeons-and-dragons. 5e does not have any functional rules mechanics (that I am aware of) that specifically are related to a tower for the wizard class.

There can be reasons why it is advantageous for wizards to live in towers: you can see much further, and blast approaching enemies with fireball before they can get close. You can have an alchemical laboratory with all kinds of stinking gases and concoctions, high above the roofs of the city, where those will be diffused by the wind and do not alienate your neighbors. And you can watch the stars from your roof, far above the fog of the surrounding countryscape.

Spells that work well with a tower shape

Even though there are no explicit class features or rules in 5e supporting wizard towers, the following spells work well in particular in combination with building a tower, and can provide an incentive.

There is the spell guards and wards that has an extra effect in stairs being filled with webs that benefits from a structure with lots of staircases, such as a tower.

And a tower has less need for windows and doors on ground level, making it cheaper to secure against less determined thieves with spells like wizard lock.

You could create a tower cheaply by casting one or multiple wall of stone spells (although I’m not sure of the static calculations - maybe need to double wall for thickness), then having carpenters come in to put in doors and staircases.

(See also: TVtropes' Mage Tower)

Answer 3

To protect the wizard, his treasure, laboratory and personal library from thieves, enemies and rivals.

The notion of the wizard's tower has appeared since early editions of D&D. The 1979 AD&D 1st editon Dungeon Masters Guide, p.103, gives the example of a player character visiting the tower of the wizard Llewellyn ap-Owen to seek his services. That sourcebook also had rules for the price of building fortifications, and the magic item called Daern's Instant Fortress.

While the wizard's tower is no doubt a thematic concept, there are several stated practical reasons mentioned in D&D sourcebooks to explain why wizards may want to inhabit a tower, rather than some less fortified residence.

• According to FR5 The Savage Frontier (1988) p.43, the archmage Ssessibil Istahvar inhabits the Lonely Tower because he wants privacy to conduct his magical research. This book notes that it's difficult for a powerful individuals to keep their location a secret, which would make them reliant on a defensible home to dissuade intruders.
• In Silver Marches (2002) p.10-11, another reason is given: Istahvar's Lonely Tower is successfully breached by a rival wizard, who steals his collection of magical treasures. Evidently, the tower was supposed to serve as a secure place for his laboratory and collection of treasures, and prevent his treasures from being stolen while he's away. This book again notes that the tower's purpose is to prevent him from being located or attacked by enemies and rivals.
• In Castle Sites p.50, the wizard Vendreth uses a four-level tower as a a base of operations for his wizard operations: it serves as home for his apprentices and a location for his laboratory, his library of books, and his magic items and wealth. The benefit of a tower is that the upper levels are protected with magic to prevent trespassers and thieves. Vendreth has also used his magic to create the tower, which would be an expensive and time-consuming task for a non-wizard, but easy for a spellcaster.
• In REF6 Rogues' Gallery, the wizard Aurora lives in a tower with an enormous library containing thousands of books. The wizard Ari Stauffan similarly lives in a tower because he doesn't like to be disturbed by visitors.
• In Wizards' Spell Compendium Volume I, the spell distant diary suggests using magic for a wizard to keep his journals in his tower for safe keeping. In other words, the wizard's home has to be defensible while the wizard is off adventuring or researching, because it contains the wizard's important and difficult to replace research notes. It has to be defensible against burglars.
• Wizards' Spell Compendium Volume III, under the spell Sathrah's Ingenious Recollection, notes that Sathrah himself died falling from his tower because he was unable to prepare a fly spell that day. Powerful wizards can enter or exit a tower using flight, which is unavailable to common thieves.
• The Al Qadim sourcebook Land of Fate p.10 notes that sorcerer's towers appear on maps as a warning to steer clear, since sorcerers value their privacy and intentionally build towers far from civilization to avoid human contact.
• According to Book of Artifacts, wizards like to sequester themselves in a private location to craft magic items in secret. The method of creating items is a secret (one might imagine that a wizard wants to keep it that way). According to Book of Artifacts p.108, wizards require a well-ventilated area to build a forge, furnace or a potion brewery. A tall tower is also useful as a site for celestial observations, which are useful to arcane processes. As per p.132, overcharging a magic item is a dangerous process which can blow up a laboratory, so again it's beneficial for this to be somewhere far from civilization.
• In College of Wizardry p.6, a set of grand towers are used as a formal wizards' academy. The towers was originally a stronghold used by powerful spellcasters in ancient times, and were selected both for historic significance and practical defensibility. There are similar towers of magic in Dragonlance.
• Stronghold Builder's Guidebook p.7 gives the example of a wizard who constructs a keep as a base of operations for an adventuring group, whom the king has tasked with defending an important local mine.

In short, according to official Dungeons & Dragons sourcebooks, powerful wizards often live in towers because they can create towers quite easily using magic, and have need of a highly defensible home for reasons of security and privacy. They have many valuable belongings which they need to protect from theft or harm, especially while they're away. They often undertake long periods of research or item crafting where they don't want to be disturbed, and often have enemies or rivals who want to attack them or steal their magical research or items. They may also wield significant personal and regional power and require a base of operations from which to control the local area. All these things necessitate a defensible home, and a tall tower or castle of some sort is a time-tested solution to this.