Why restrict it to one type? I always thought of forts as being to some degree modular, with different things that you can put on in edifferent combinations.

I think this is a great idea.

Based on the what I understand to be the philosophy of the game. It could be implemented by creating an upgradeable "fort/tower" or defensive building type. The buidling itself would have no name, maybe some basic defensive value, and would have a limited number of upgrade slots for you to choose your specialization.

Maybe similar to GC2 you could have to ability to build larger building types in the future with more upgrade slot capabilities or better upgrades which you could place on the building.

It might also behove the game balance issues to have to start newer, larger, defensive structures from scratch instead of limitlessly upgrading the same "fort/tower" building.

To take it to an extreme, build castle design into it ala the ancient "Castles" game.

Or to be more reasonable, I love the modular upgrades thing, and I like the possibility that turtling could become a viable strategy to some extent. After all, fortresses had massive strategic significance in medieval times.

I'd love to be able to build customized defenses that got used on the battle map.  Different preset types would be acceptable, but you should be doing a lot better than stat changes when you have tactical combat available.

  That removes not only the visual variations since by mid_game all major players will be using the wizards tower and giant castle combo, but it also sends us back to using a linear path where one or two types are superior.  Each type of fort should provide unique advantages which can be gradually improved so there's more strategic paths.

A city with barracks and a strong wall would be rated as the default option_1 with no advantages beyond what's provided from the wall and the building type.  Once one of the fort types listed in the original post have been constructed the player would receive specific advantages which can be incorporated into a larger strategy.   The list of fort types can be made available from the "center" tile.

The visual benefit is that most enemy capitals will be different which is more visually appealing then having all enemy capitals look like one of two choices.  Based on screenshots from the journals I believe we currently only have a good(life) and evil(death) capital image.  The fort types could also be modded where gamers could add their own new forts with new images and benefits.  The strategic benefit allows for specific strategic benefits for each location which can be part of a larger strategy.   Naturally how the tiles for each city will also influence the location as well.  

What about building towers, forts, etc. outside of cities? I was hoping that you could build one in the pass through which enemies might easily come. That way you have the bonuses the tower grants? I guess this is similar to Starbases in GC2. 

Additionally, maybe walls could combined with towers as well. Think great wall of China. Not cheap to build, but it would give you a great tactical advantage or protect your small peaceful kingdom against the larger aggressive enemies.

The city evolution would be separate from the optional fort types, thus if someone wanted a specific type of fort for specific advantages it would be the choice of the sovereign.  This allows for more strategic combinations such as a village with a citadel, a city with a wizards tower, etc., etc., .  The "home" tile would only be the location where the fort type is purchased for construction inside the game. 

All great ideas which we can eventually test and adjust for game balance during beta testing if  Stardock is interested with this idea.

Well that's why it would need to be tested during the beta phase for gameplay balance.  All the bonus examples are just arbitrary guesses... until we begin playing the tactical battles and acquiring the common resources we won't know what bonuses will be fair and balanced for each structure.  At this point we can't even place a cost on the structures or an estimated building time. 

I like the more modular idea, it seems more fitting with the flexibility that Elemental seems to be shooting for.

The semi-random list in the OP is interesting, but I'm pretty sure it would be more fun if it was modular and not incremental.

Unless maybe you changed the verb 'upgrade' to something like 'remodel?' I don't see a clean upgrade path in the draft bonuses and I imagine I'd be very annoyed at having to somehow give up a line of sight bonus because I'd incorporated my skinny little tower into a sturdy basic town wall.

Hmph. I'd probably still want a modular approach. Something like that could help distinguish major, growing population centers from things like frontier garrisons and farming villages. Kinda seems like you should need to use up more land to gain substantial fortifcation bonuses, at least if you want to avoid interefering with whatever non-military activity is going on at the site.

The modular approach is best in my view.

All types of buildings could be added and upgraded.

just a few additions:

Moat

Bailey

Square towers

Round towers

etc.

Lots of interesting info here