Let me lead off with a disclaimer … yes, I know we're playing a fantasy game.

I prefer that my worlds are mostly plausible so I make sure when I develop a town there is an economically viable reason a town would arise in that spot.  I make sure there is adequate food and water to support more people than the number living there. And blah, blah, blah ...

So I need to design a wizard's tower for the central town in a campaign.  Oh, did I mention that I work in the building industry so I have a pretty good sense what is plausible.  So I want the wizard's tower to be functional and cool.  I want it to have a way of having hidden rooms and stuff.  This is not so easy when you build a big cylinder from the ground up.  I may need to apply an extra dimensional door to pull off that part, but we'll see.  But in general I want the tower to make sense.

A tower should not have a height greater than 5x the base.  A tower that has a slight taper as it rises is more plausible than one that goes straight up, but it is harder to make.  However, the blessing of fantasy let's me ignore that one.  I did an Internet search and came up with rough sizes of several bell towers in Ireland and I think I'm going to use that as a guide.  My Wizard's Tower with have a base of 40' diameter and rise to the height of 144' at the top of the peaked roof.  The top floor elevation will be 120'.  There will be a smaller round tower extending another three floors up from 120' and this will end in a flat roof an a ring of battlements for guards to use as a watch tower also.

Inside the tower the wizard will have three floors dedicated to housing space.  One living space, one kitchen and eating space and one sleeping space.  The guards will have the lowest three floors.  The third floor is a guards mess.  The second floor is a general storage space.  The first floor is an armory.   The top floors will be for the wizard's profession.  The tenth (top) floor will be an observatory, the ninth floor will be a library and scriptorium, the eighth floor will be a laboratory space, and there will be one level for mundane storage for the wizard.

What cool ideas do you have for a wizard's tower that I should consider?  I want to make this an adventure just to explore the tower.  Thanks for your help.

I think the tower should have a magically secured vault, where the wizard stores artifacts and a spellbook copy.

That would make sense from the wizard's point of view and be a tempting target for adventurers.

Okay. Couple things. 

First, thanks to the 8th level spell "Demiplane", the wizard could have access to a 30' X 30' room anywhere he wants one.  He might have one, or ten, or fifty of these rooms all over the tower. These could be used for storing some of the more dangerous material components of his research, or just to keep the mundane clutter out of the way. 

And the 7th level spell "Magnificent Mansion" creates a doorway that leads to an extraplanar space of 5,000 square feet, and it's already furnished! And it has spectral servants at your command! So, a few castings of that could more than double the square footage of the entire tower.

Second, just because the tower is visible above ground doesn't mean that it won't also extend below ground. After all, a 14 story stone tower will require a substantial foundation to prevent a Pisa Problem. So maybe the basement/foundation leads into an entire underground dungeon system. That's just free real estate! 

And finally, remember to factor in defenses. Sure, this wizard is probably close to 20th level, and the players might be starting at Level One, but you just know... you just know... you're gonna need some defenses built into this thing. "Seeming", "Guards and Wards", "Walls of Force", "Private Sanctum", "Glyphs of Warding", monsters, golems, maybe even a pet mimic. Whatever it takes to player-proof the tower the way a new parent would baby-proof their house. 

This is silly, but I put it in a campaign that I designed once. Enchant two doors into a room so that every time one of them is opened the other one closes if it is open. It doesn’t do anything other than add a touch of whimsy to the room, but don’t wizards spend time experimenting?

Have you looked at Galder’s Tower?

The door is actually 20 feet of the ground and they levitate or have unseen servant pull a ladder up behind them.

Note, actual monastery towers did this in Ireland so when the vikings raided them, they could stash the good loot in the tower and then pull up the ladder.

As others pointed out, the biggest feature to consider about the tower is that a wizard lives in it. The D&D spell list is full of spells that players might find useful but I would be surprised if wizards did not have a long list of more mundane spells to make life more convenient.  How about enchanting pipes to make them move water without a pump?  Making stones lighter and stronger? Doors that will take you to any floor with the right key?

Is there a reason for the wizard to have a mundane tower instead of a magical one?

If not, go nuts - architectural integrity (or mere tastefulness) be darned. Many Wizards have towers "just because I can" and the tower itself reflects their power (and often lack of tact).

If it needs to be mundane for a reason, it will make an interesting surprise for adventurers to realize that a wizard occupies the tower when it seems so normal. A wizard that has a tower but doesn't use it to flaunt the power the wizard has? Unheard of, but that makes it interesting.

The tower is in a prominent position on the edge of a city-state walled town.  On the other side of town dwarves are working an iron mine and have built a temple to their forge god.  I am working on a back story for the tower where the dwarves built the wizard's tower also.  On the whole, I prefer that the tower be built in a mundane fashion, but I intend to use some of the ideas above about water pipes and stuff.

The wizard will have his wife living in the tower with him.  She will have a maidservant and he will have three servants; one a laborer, one a scribe and one an apprentice.  They will live in other quarters, but I suppose I could put them in the first basement level.

I had decided to add two or three basement levels and that is where I will create the "hidden" room, with the DMs concurrence.

Thanks for the ideas.  I find this very helpful.

i think you're creating a tower for a noble who has a pet wizard.  If it's a wizard's tower, get rid of the peon guards. also forget the servants. it's a wizard's tower, that's what you have spells for.

Eltz castle in Germany but narrower and taller otherwise use magic to make axillary rooms with and have the tower be a normal tower protected by magic. Like the spell: forbiddance to protect from teleportation in, Guard and wards to stop intruders, hallow against undead or etc if they aren’t a necromancer(cleric spell), Mordenkainen’s Private Sanctum for the personal area. 

Demiplane only lasts 1 hour so that is good for backing up a spell book and supplies but no use outside of that for a d&d story with the way the spell is written in 5e.

There are other tall castles but I still recommend the aesthetic Of Eltz castle. Also having portals that link to other locations and wacky inside areas because of magic. Thief: The Dark Project has a mission with a mages house.  

One idea would be disconnected floors. There is no access outside of magical access to a floor (be that to teleport in, or to step through a mirror or door elsewhere in the tower). This works both as a security feature as well as a space saver :) No need for big stairwells taking up space. 

Going under the tower also creates some additional spaces for bulkier things like storage of those most volatile spell components and testing chambers for spells. 

It seems almost too obvious in a way - but a library of some wonder is a must for a wizard. Several others have already mentioned things like pocket planes and additional connections to demi planes. However about enchanted bookshelves that reach through to an even bigger collection (bookshelf of holding?)

And just because you are a wizard doesn't mean you don't want comforts in life. Don't worry about the plumbing for that huge marble bath when you can have an enchanted pipe pull water water directly from the plane of water, or that fireplace that seemingly never needs wood because the fire comes from elsewhere. 

A thought that occurred while reading the initial post: a good way to conceal "hidden" areas in this tower would be, if say: it had an elevator system (whether mundane or magical); the "same floor" could in fact contain multiple floors, or indeed "fraction" floors ala Platform 9 3/4; accessed via keys or other such methods without having the problem of having to find a way to deal with extra-dimensional space conflicting with a more conventional stairwell system or other.  Just an odd idea my brain pinged onto.

Another thing to remember about a magic-heavy setting: magical metallurgy and materials would also be a thing to consider: granting access to materials with super-natural tensile strength or other unusual properties to exploit when designing a structure.

This is a cool topic.

On thing to remember (but considering you work in construction you probably already know) is to make sure the building is actually big enough to support itself. The floors themselves actually take up volume and even though most houses had lower ceilings back in "the olden days" (even though many castles and similar places had fairly high ceilings) you still need room for the actual beams and structure of each floor. Another thing to consider is the area of each floor. Sometimes people forget exactly how small a 5x5 feet square really is. You want to make sure you have room for stuff in your tower. :P

A summoning room, would be a good idea, where it contain an arcane circle 

A divination room with a scrying device such as a magic pool or crystal ball. 

A construct room where a golem is half finished.

A time and gravity room, where the flow can be altered in different ways.

A portal room that contain a gate to several planes. or other crystal spheres and a teleportation circle that can teleport to other place in the prime material world. 

Along with the wizard's familiar, it could have unseen servants and guardians, perhaps golems or animated armors and weapons. 

I believe that you should make a whole underground dungeon system for storing treasure and dangerous experiments, but its your design, make it however you want.

Towers are great. Also great is their subterranean dungeon. IMO the dungeon sq ft should be much greater than the tower sq ft.

General tower and dungeon features:

• use of extradimensional space
• portals that teleport you between the rooms, tower and dungeon, etc.
• protection from divination in as many rooms as possible
• lots of magical and non-magical hidden doors and traps
• lots of security (e.g., alarm spells, arcane locks, etc.)

Beyond the ordinary bedroom, guest/apprentice room, kitchen, restroom, I'd have:

• study/library with books, scrolls, journals
• lab for alchemy with extra material spell components
• teleportation room (top floor)
• summoning room
• divination room

Dungeon:

• magical items room in dungeon
• (magical) prison for holding enemies, thieves, etc.

Gated surrounding outdoors: garden (homegrown alchemy ingredients, material spell components that can be grown, etc.) with some aggressive plants/vines :)

Why does it have to be a round tower?

It could be anything in that town.

How about a windmill? The dwarves used to use it for the old crumbling forge works next door to it. They eventually built bigger and gave it away.

If you have the cash you could very easily have servants to do the mundane tasks, Why waste your time and spell effort to clean the windows and your stanky clothing.
Everything above ground is normal and mundane but you could have a underground area hidden from everyone where you keep all the good stuff like your real magic library.

The windmill could still be used as a grain mill to keep a small income to pay for the household and its staff.

not that the discussion needs it, but a tower doesn't have to be a lived-in structure. there's also historical precedence for a tall, hollow stone structure allowing air heated by the sun to be pulled up and out the open top which then draws in cool air through the doors and windows below. or windcatchers that forced air to travel into and down through one side while it also exhausted the forced air up the other side, hottest air first (thus leaving the inside breezy and marginally cooler, even in a desert environment). and then there's the previously mentioned dwarves and their forge: smoke has to get out somehow.

any of those chimneys makes for a non-wizard reason to have a tower with no doors / windows at ground level. and each could easily be attributed to / attached to a subterranean system if you like. and even then, a wizard (or other) might happily repurpose one of those disused chimneys and just expect traps to keep the underdark from creeping in. or, even better, inhabit a working (smoke) chimney as a dangerous central staircase connecting safe inter-dimensional rooms on each floor. all of which assumes the redecorating wizard is looking for stealth or seclusion. if you're just asking for a main road accessible functional fort shaped like an upright pencil, then i think all the earlier comments have it covered.

Quote from Houligan >>

The door is actually 20 feet of the ground and they levitate or have unseen servant pull a ladder up behind them.

Note, actual monastery towers did this in Ireland so when the vikings raided them, they could stash the good loot in the tower and then pull up the ladder.

 This is a petrified Rocket; the op was asking for ideas for a tower, silly.