Here's where I'd use each structure: The wall is tricky. The wall should never be put around your entire town. Instead, it would be better to use it to guard a crucial building, like a Wonder or Titan Gate. The reasoning for this is that with any wall, just one tiny segment gone, and it's useless. So it would be easier to make it smaller, or you'll waste time sending villies to repair it every time you get sieged. At least they look cool in scenarios! Fortresses should be built also for getting specialty units like the portable ram or the elephant. Also, some minor gods provide special upgrades and benefits that have to be researched at the fortress. The fortress is very useful at chokepoints. Research the Levy upgrades, and you can quickly get an army to defend against a boom. Besides defending against resource raids, Towers have a second use that can help if you micro. You can detect a boom with them. Build them a ways from your town, and when they get attacked by the boom army, start building an army/fortress. You can also vortex (if atl.) or Shifting Sands (if eggy) to the tower to attack with your existing troops, then build more should there be "leftover" to attack your town. Towers could benefit from having a Carnivoria placed next to them too. And of course, they provide the uber-important Boiling Oil upgrade. Feedback?
Cheap +1/2
Easy-to-build +1/2
Can Research LOS boosts +1/2
Can Research Boiling Oil +1/2
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Weak -1/2
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TOTAL:
Flexible +1/2
Cheap +1/2
Easy-to-build +1/2
Denies Complete Access to Area +1
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Tedious to Manage -1
(Sometimes) Weak -1/2
Long Walls = Hard to Build -1/2
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TOTAL:
Can Build Unique Units +1
Sturdy and Strong +1
Good LOS +1/2
Powerful For Town Defensive Function +1/2
Some Minor God Imps. Researched Here +1/2
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Costly -1/2
Takes Awhile to Build -1/2
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TOTAL:
Therefore, for strategy purposes, it appears to Fortress is your best bet for a place that'll be around for awhile. (i.e. main town, forward base) The tower is useful to defend a temporary area. Useful for raid deflection.