To my dear DJ Fuzion,
Tower defense... I love the genre very much. Perfect mix of old school arcade gameplay with modern obsessions like upgrades/skill trees. As such I've played a wide range of the type and have noticed things that I distinctly love/hate about the genre.
Let's see what we can do here.
Are you aware of "Symphonic TD"? - kongregate.com/games/Fr0z3nf1r3/symphonic-td
It's the game that most closely resembles what you're going after, and in my opinion, a study on "what not to do". Personally, I find this game to be... eh, ugly. The interface requires you to click more buttons than is probably necessary (what's with all the sub menus here?) and requires you to dash from hard right edge of the screen to the center for object placement to the top to start a wave. Without the forced tutorial at the beginning, this "game" more closely represents "software"...
It uses music NOTES as enemies, and as the enemies appear, they "play" a note for the user to hear. I think music notes as enemies are rather drab... people would probably prefer a more CREATIVE VISUALIZATION of their music as opposed to blasting away unidentifiable (for most people in the world) runes. The towers themselves are (basically) color bricks... again, not much there that's very inspired.
It would appear, at this point, that "music" and "tower defense" don't go so well together.
POINT OF REFERENCE TWO: "REZ" or "REZ HD" (Sega Dreamcast original, found on Xbox Live Arcade and PS3 Network these days.) - youtube.com/watch
Rez is a complete musical visualization of a hacker forcing his way onto various secure networks around the world. You see how he has a reticle that raises in number? Players press and hold the shoot button, and drag the reticle over numerous enemies (up to 9) and then RELEASES the button to unleash a wave of attack. The background music is a basic loop... and all of the player ATTACKS create musical cues. Even if all you do is tap tap tap the button, you hear a slight high hat/cymbal sound.
As each wave advances, the music pumps up in intensity. The number of enemies increase, and the amount of musical contribution by the PLAYER is increased. With the simple looping tunes going on, players can create just about any hodge podge of notes and have it come out sounding pretty trippy.
More on this later in a moment.
Another Rez Example - youtube.com/watch
What's most important here is what kind of sounds the (towers, in your game's case) create in relation to the music. I'll tell you what's a nice convention in a few tower defense titles, that you can start with a basic "type" of tower and then upgrade it out for, say, elemental damage. A fairly basic example of this is "Arrow Tower" can, after three normal base upgrades, be designated as a "Fire Arrow" tower or a "Piercing Arrow Tower". Fire Arrow will strike an enemy and deal burning damage for a few seconds thereafter, increasing the basic shot's potency... although you get just base damage from the shot if your enemy happens to have fire resistance. The "Piercing Arrow Tower" will deal no elemental damage, and ignores enemy armor (physical damage resistance) and has a chance at delivering a critical hit which would be 2x base damage. Two different types of effects from the same tower.
Therefore: I propose a similar system for you. As you are creating a musical game, it might be nice to have a smaller set of basic towers with a wider range of effects, player driven. That way a player has the opportunity to be more involved in the "music creation" part of gameplay, and you (overall) have less work to do outside of sampling different instruments.
These towers are listed in order of ascending value (cheapest to most expensive) based on effect.
LVL01-03: Acoustic Guitar. Deals basic physical damage.
LVL04: Electric Guitar or Bass Guitar tree selection.
LVL04-06 (ELECTRIC TREE): Faster rate of fire, electrical damage, chance of shocking enemies (Freezing them in place for a second or longer).
LVL04-06 (BASS TREE): Rate of fire proportionate to level (though never reaching the speed of Electric Tree), stronger physical damage (referred to as reverberating damage?), critical hit chance.
Drum Kit Tower
LVL01-03: Snare Drum. Fast rate of fire, though proportionately weaker than the Guitar Tower. Better suited to handling clusters of weak enemies.
LVL04: Cymbals or Tom Tom tree selection.
LVL04-06 (CYMBALS TREE): Ignores physical resistance to deliver fast, consistent damage. Has a chance to chain attacks across multiple enemies (again, probably the equalizer for clustered groups of attackers.)
LVL04-06 (TOM TOM TREE): Somewhat slower rate of fire, deals reverberating damage, chance to cause headaches (which effectively makes this your Ice/Slow Tower).
LVL01-03: Standard Piano. Rate of fire lays between Drums and Guitar. Because of the number of keys involved, I imagine this one would be best suited as a "spread fire" tower, and thus is able to target multiple enemies/should be more expensive to produce than the other two basic types. Let's say for now it can target three enemies per shot in it's range.
LVL04: Grand Piano or MIDI tree selection.
LVL04-06 (GRAND PIANO TREE): Allows piano to target four, five, and six enemies per shot in its range (based on level). No critical hit chance, the level upgrade comes from the fact that it spreads out it's attack so deep whereas others have a small chance of chaining or otherwise single-target attacking.
LVL04-06 (MIDI TREE): Will only ever attack three enemies at a time, but as level increases so does the chance to inflict "Confusion" with it's futuristic sound. "Confusion" will make enemies reverse their direction momentarily, sending them back distances increasing per level to walk through towers all over again. Since it doesn't get a target multiplier bonus, qualifies for somewhat higher rate of fire than the Grand Piano tree.
LVL01-03: Clapping hands, mixed male/female humming? Slow rate of fire, because multiple people voices sound like a mess compared to multiple guitar chords, drum hits, etc. View this as a "rocket launcher" type of tower. Small splash damage impact radius.
LVL04: MALE or FEMALE voice selection.
LVL04-06 (MALE VOICE TREE): Random selection of "Yeah!" or "Woo!" "Uh-Huh" or what have you. Slow rate of fire, but upon impact, has a wide damage radius (Splash damage). Has a chance to "Rock" (Critical hit) enemies for 3x damage in the blast radius... signified by a 1980's glam rock wail.
LVL04-06 (FEMALE VOICE TREE): Random female vocals, as above. Somewhat faster rate of fire, somewhat larger splash damage radius, but has a chance to "Harmonize" all other towers in it's range, allowing ALL tower types involved a brief window where ALL attacks dealt do 2x damage. (People will be inclined to build lots of female voice towers in proximity to their guitar, drum, piano towers etc.)
---And now... a look at some "Accessory Towers". They have no branching trees, are the most expensive to build, but have other benefits for players that can't be ignored. They are listed according to what I feel should be the cheapest to buy up to the most expensive, based on their benefits.---
LVL01-03: Damage amplification for all towers within it's radius. 2x, 3x, and 4x boost to damage, based on level. Effects DO carry over to critical hits, but DO NOT carry over to status-based attacks (Slow, Reverse, Shock-Lock, etc.) Does zero damage to enemies.
LVL01-03: Rate of fire increase for all towers within it's radius. 33.3%, 66.6%, 100% boost based on level. Provides NO boost to damage for towers, however, as the rate of fire increases so does the overall chance of status effects. Does zero damage to enemies.
Fan Club Tower
LVL01-03: Reduces upgrade costs for towers placed within it's (wide) radius. 15%, 30%, 45% reduction in upgrade prices. Also boosts the resale value of towers by 15%, 30%, 45%, based on level.
Record Executive Tower
LVL01-03: The Record Executive's job is to double, triple, and quadruple the money earned by surrounding towers. With each level up, the radius is increased slightly. Deals zero damage to enemies.
--PROJECTILES! How do I visualize these weapons?!--
Why, with COLORS silly! Don't have freakin' music notes flying out, or crazy, hard-to-animate waveforms, etc. I would imagine THIN lines that appear vibrantly and then fade out gradually is the best way to go, with the lines getting THICKER as the towers level up. This provides a visual feedback for players to tell where their strongest towers are, and where (if any) there are towers needing upgrades. In later waves, with more towers, you'll be creating quite the light show...
Have a look at an example of what colored lines can do for your visualization: youtube.com/watch
Trust me on the "colors in straight lines" thing. CERTAIN GAMES MADE QUITE A KILLING FOLLOWING THIS FORMULA! You may have heard of this ONE little indie game production from a fledgling production studio that nobody remembers... youtube.com/watch
Take some time to read up and study on "Color Harmonies", that is, what colors go together, and what colors stand out AGAINST each other very well. - tigercolor.com/color-lab/color-theory/color-harmonies.htm
Think in terms of:
Guitar Tower should have three colors that are Analogous to each other, but Guitar Tower, Drum Tower, Piano Tower, Artist Tower should be placed Square to each other, each respective tower following the Analogous color scheme.
--Enemies: Keeping your Friends Close--
Why they gotta be enemies? Why can't they just be fans of varying genres? I imagine a ball, painted up to look like a member of the KISS army to be called a "Metal Head"... and they have immunity to the ELECTRIC GUITAR tree, but are susceptible to the PIANO tree (as it's not considered Metal). For reference - 3.bp.blogspot.com/-M6fIsY5XaB0/UQZ9pPTyT5I/AAAAAAABrRw/cG1jVjxq1Ww/s1600/kiss-tour_loch1.jpg
You can introduce "Ravers" that are immune to the MIDI tree, "Headbangers" that are immune to Drums, etc etc. Hell if you design these little "angry birds-esque" balls of fandom just right, you might even accidentally stumble on a line of merchandising for yourself.
...read this over, let me know what you think. We can put more stuff out there for you if there's still questions.