Back on track... a highly visible construction would serve as a lightning road for visitors to your fort. Even people that weren't looking for your fort/those that were lost could see the tower.
Rationally, that would presume there was already enough traffic close to your fortress that would put them within line of sight. Keep in mind that we're frequently talking distances of several day's travel/dozens of km between sites.
It doesn't matter how big the landmark, people aren't likely to go visiting close enough to the Bristles of Witches, a terrifying swamp, to see your 30z-tall solid gold warhammer statue.
Comparatively, it should be relatively easy to see your fort if you're built directly on the shortest path (or you help construct roads to MAKE it the shortest path) between two human civs.
In general,
as was being discussed in the Hidden Fortress thread, what happens on the worldgen level should be much more notable than what happens in your fortress, itself. A tower is notable, yes, but you'd see the large wagon ruts from all the caravans going to your fortress from much further away.
The only other thing I can think of is that the tower would attract more aerial monsters (harpies, drakes), but for this to be worthwhile they'd need to bring a unique/highly valuable resource or otherwise fulfill some sort of requirement or other fortress milestone... but those would be a -very- long way away from this alpha version of DF.
You can already capture rocs and dragons and breed them if you're wildly lucky enough to get a breeding pair. (Or at least, tame the females, and have "really big omelette night" for your dwarves. Note, roc eggs are three times the size of the typical adult dwarf.)
In fact, if towers attracted rocs, and you just set up some cages to catch them, they're practically the best pets ever. They egg lay, are immortal, are huge (450-900 meat and bones!), and can be trained into war rocs AND can be used as an exotic mount. (Which means eventually having an aerial corps of elite marksdwarves on roc-back. BWAAAHAHAHAHAHA!)
Also, harpies already exist in evil swamps and grasslands, but they're not as interesting or dangerous as a giant eagle, anyway, so it's kind of a letdown. They also cannot lay eggs, be tamed, or butchered, so they're like mermaids, but more boring.
I'm mostly thinking about rare alchemical ingredients (harpy feathers, wyvern stinger) or using drake/dragon scales in armor construction. Their fire-proof nature would actually have to matter, and we'd naturally have to be able make armor out of scales (and have it be stronger than normal leather). It could be a useful iron/steel alternative though. "Couldn't find any iron below, better build the tower, chain the elves and dogs up top as bait, and hope the drakes/dragons swing by."
Dragon and fire imp leather already is immune to fire and magma. It's just not any better than normal leather, currently, because all other stats use the same copy-pasted data.
Invading Heaven though... it is going to be worse than breaching hell. Heaven includes Archangels that are the size of/larger than FBs, have grandmaster level in all combat skills, and are armed and armored with metals better than steel (but not candy). The soldiers have the same divine metal equipment but lesser stats. They are still huge and have massive attribute boosts though.
But I don't think angels have HFS-style perks, which would mean they "fight fair", so they'd probably be easier to take on with a straight military. (I.E. no solid steel blobs, no flying webbers pairing up with flaming projectile spitters, no deadly dust, etc.)