Well, take heart in this; you at least realised your buildings looked bland. Most people don't.
Are you on Planehacked at all? If so come visit my island. I work mostly with stone brick, and only in default texture, so my solutions should apply to your structure quite well.
If your not, I'll post some pics once I get home. That said, I run on 3 simple rules;
Vary the shape;
Vary the texture,
Tell a story.
So taking those into consideration;
Shape) For a tower, I find things like a wide base, narrow shaft, then a slight bulge at the top generally works well for medieval structures. They did not have the incredible stength of things like reinforced concrete; building with stone meant broad, solid foundations, and very small overhangs (stone does not handle tension well). Space is achieved by using arches, which keeps the stone under compression. Slopes and arches can also be used to draw the eye toward the centre of the structure, rather than having straight cardinal lines that lead away from it. It is also a good idea to have minor focal points on the structure itself (not just the tip, but on the way up as well). Make each individual part have some point that acts as a focus, with the immediate surrounds complementing it.
Texture) Texture is important, and probably the most overlooked part. The most amazing castles in MC will still look like crap if they're pure cobble. Think about where it would be used in real life. In RL cobble might be used to infill much of a wall, but it would be bordered and interspersed with pillars of dressed stone to give it the strength and cohesion it would otherwise lack.
Contrast and complement is important; general rule of thumb is majority complement, minority contrast. If a structure is primarily dark (e.g. cobble), add small amounts (accents) in a contrasting, lighter tone (smoothbrick). If it's predominantly a warm shade (e.g. sandstone), give it cool accents (lapis). If it's generally a complicated texture on a block, intersperse with a simple one.
Make sure you use accents sparingly though; if there is too much of the contrasting block, it will look garish, so instead keep within similar tones/hues for most of the structure.
Majority complement, minority contrast.
Finally, Story) Give the structure a history, and build to a purpose. Is it a lonely frontier fort, surrounded by hostile enemies? Then it sure as hell wont have a great big front door and plate glass windows. It will be small, tough, and built from easy to obtain materials (raw wooden logs, and cobble, not lapis and gold). By contrast, a holiday home in the peaceful countryside won't come with a ballista in the front garden, and will be built with worked materials (polished floorboards and dressed stone).
So, for a wizard's castle, how did he make it? Is it a ruin he fixed up on his own? Did he hire villagers to build it for him, or summon it from the very earth itself? Why did he make it? Is it a place of meditation and contemplation, designed to keep out the raging mob, or contain an unspeakable evil? Does he have the funds and time to maintain it, or is it being slowly reclaimed by nature?
Given you're wanting a chunk straddling tower, you're stuck with square and blocky. That says to me; old castle that he has reclaimed. So, you'd want solid foundations to resist siege. Lower floors will have very few, if any windows, and those will be little more than arrow slits (use an inverted stair atop a regular one for those!), and the roof will be overhanging ramparts to prevent enemies climbing over (also keeps spiders out
).
How did he get it? Probably abandoned, so while the solid bottom floors will be intact, upper floors may be crumbling, with holes in the walls (cobble blocks and stairs next to cracked smooth brick and a layer of cobble scree near collapsed walls can give the impression of stone decaying away due to weathering).
There would probably be a few outbuildings around it, an old kitchen or privy. These would most likely be wood, and so long since decayed into little more than sone foundations and mounds of grassy dirt. Long grass can greatly help a place look overgrown too.
As with all things, references will also help. Google examples of real castles and artists' renditions of wizards towers.