Would it be an exaggeration to say that iTunes built the legal music download world? Maybe not - after all, it has 85% of the digital music download market (which is way too much for the entertainment industry's comfort). Apple is the undisputed king of this world. And there are upstarts who want to revolt - Microsoft, Sony, SpiralFrog and a few other brave souls. But a Financial Times analyst reckons they're going to have a heck of a hard time scaling Fortress iTunes' walls.

What is the secret to the success - nay, dominance - of iTunes? The analyst chalks it up to rivals botching up their plans to compete with Apple, but more to key advantages Apple holds in the market.

• First, they've got network effects and technological lock-in on their side. People can share their iTunes-downloaded music with each other's iPods - there's the net. And Apple's digital rights management software (so far) allows this to happen - there's the tech. These two turn the iPod and iTunes into natural partners, feeding off each other's market advantages and popularity. The word for this is "synergy."

• Not one to rest on their laurels, Apple has diversified and segmented their captive market to squeeze out more profits and an even bigger share. There's the video iPod and downloadable videos from iTunes. There's the basic iPod, the Mini, the Shuffle, and the Nano. And there's all the marketing support the company has poured into the iPod, turning it into a style icon and a hot-buy item.

• And finally, perhaps the greatest advantage of all: it pays to be first, it pays to be ahead. History demonstrates that, once established, technology leaders are seldom overthrown by direct competition. No, to seize the throne, you've got to go for the jugular: the business model, the technology paradigm. In the case of Apple, the iPod-iTunes synergy.

Not surprisingly, one rival has indeed gone for the jugular: SpiralFrog. Their secret weapon: free media downloads (as opposed to iTunes' pay-to-download), supported by advertising content tied to the download software. But the rust in their sword? An analyst thinks that an ad-supported download service is quite a stretch. This is because - despite Universal Music Group's decision to join SpiralFrog - other music companies may feel that simply giving their music away would risk devaluing their product, ad or no ad. Besides, users who want free music still turn to (often illegal) peer-to-peer networks.

And worse yet, there's still DRM to worry about. As we reported before, SpiralFrog downloads won't run on iPods - which locks out a huge number of iPod users. And now, we've learned, users would have to return once a month to refresh or renew their music downloads.

Aside from this stab at the heart, there are three other avenues to challenge Fortress iTunes.

• Outflank. Find different ways to introduce and distribute new music (e.g., MySpace, MTV's Urge).
• Outclass. Challenge the iPod side of the synergy with a... er, "snazzier" product. The PSP's widescreen, for example, is perfect for movies, far more suited to the format than the iPod.
• Cover all bases. Combines aspects of the other two avenues (outflank and outclass) by introducing both product and network. For example, music downloaded over cellular networks into mp3-capable phones is big business in East Asia, where multimedia cellulars tends to be the hot-buy item.

And finally, imitation is still the sincerest form of flattery (once all the legal copyright issues have been worked out). Microsoft has the Zune coming out; Sony its Connect. All these attempts to challenge Apple, though, face the same big obstacle: the fruit company's still the undisputed king of the chessboard. And it has a big lead over all its rivals - the greater popularity of the iPod family, the established and stable iTunes model, continued commitment to maintaining that lead.

In time, even these challengers may nibble away at Apple's lead. Which would be a relief for the entertainment industry, which would prefer a more competitive download market. But for now, that fortress ain't coming down.