The citadel upgrades are meant to be viable alternative to demigod development. However, as of right now they are way too weak because they either do not scale properly, or are too expensive for what they accomplish. Here are my suggestions for improvement, along with a few new ideas.

Overall Change: For each rank invested in buildings, the reward the enemy demigods get by destroying it goes up by 40g. For instance, if you have fortified structure rank 2, enchanted towers rank 1, and building firepower rank 1, a total of 4 ranks makes that 40%, and the bonus goes up from 300g per building to 460g per building. This means that the stronger the building is, the more rewarding it is to destroy it.

Fortified Structure:

Rank 1: +10% health, +10hp per second - 800g

Rank 2: +25% health, +25hp per second - 1600g

Rank 3: +40% health, +40hp per second - 2400g

Rank 4: +55% health, +55hp per second - 3200g

Reasoning: The current hp/sec values are 10, 15, 20, 25 which definitely are not good enough. Also make the price cheaper to make the upgrades more attractive.

Building Firepower:

Rank 1: Buildings gain 15% damage and 2 range - 1000g

Rank 2: Buildings gain 35% damage and 3 range - 2000g

Rank 3: Builduings gain 60% damage and 4 range - 3000g

Rank 4: Buildings gain 90% damage and 5 range - 4000g

Reasoning: removed the splash damage component because it's not really useful, and replaced it with range which should help with the area of control that towers/forts provide.

Enchanted Towers: *NEW*

Rank 1: Building attacks slow the enemy movement and attack speed by 5% for 3 seconds - 1500g

Rank 2: Building attacks slow the enemy movement and attack speed by 11% for 3.5 seconds - 2300g

Rank 3: Building attacks slow the enemy movement and attack speed by 18% for 4 seconds - 3200g

Rank 4: Building attacks slow the enemy movement and attack speed by 25% for 5 seconds - 4300g

Reasoning: Add more utility to towers and make them harder to destroy. Combined with the previous two upgrades, the buildings should scale with demigods if they are invested in.

Currency:

Rank 1: Increases gold income of the team by 4 gold per second - 1800g (2.5 minutes)

Rank 2: Increases the gold income of the team by 9 gold per second - 2700g (3 minutes)

Rank 3: Increases the gold income of the team by 15 gold per second - 3600g (3.3 minutes)

Reasoning: Made rank 2 and rank 3 more effective because otherwise they are not really worth the gold investment, since the income bonus is not additive. The minutes in parantheses indicate the number of time it takes for the upgrade to pay for itself for a team of 3. Formula: (cost / additional gold income) / number of teammates

Blacksmith:

Rank 1: Reinforcements gain 20% damage, 15% health, and 20% armor - 1500g

Rank 2: Reinforcements gain 40% damage, 30% health, and 40% armor - 2400g

Rank 3: Reinforcements gain 60% damage, 45% health, and 60% armor - 3400g

Rank 4: Reinforcements gain 80% damage, 60% health, and 80% armor - 4500g

Rank 5: Reinforcements gain 100% damage, 80% health, and 100% armor - 5600g

Reasoning: Combined Blacksmith and Armory categories into one category, since they seem to have the same purpose (making creeps stronger). The upgrade is sufficiently powerful now so that people don't have to wait until giants to buy it.

Experience: renamed to "Introspection"

Rank 1: Experience earned by demigods increased by 10%, and gold earned by demigods when killing creeps is increased by 4% - 1200g

Rank 2: Experience earned by demigods increased by 20%, and gold earned by demigods when killing creeps is increased by 8% - 2000g

Rank 3: Experience earned by demigods increased by 30%, and gold earned by demigods when killing creeps is increased by 12% - 3000g

Rank 4: Removed.

Reasoning: Made it a little bit cheaper. Removed rank 4 since by the time you might want to buy it, you're probably already close to level 20. Added the gold element so that it pays for itself over time, albeit not completely.

Graveyard: changed

Rank 1: Death penalty reduced by 10%, gold and experience received by enemies upon killing you is reduced by 5% - 1200g

Rank 2: Death penalty reduced by 20%, gold and experience received by enemies upon killing you is reduced by 12% - 2000g

Rank 3: Death penalty reduced by 30%, gold and experience received by enemies upon killing you is reduced by 20% - 3000g

Rank 4: Death penalty reduced by 40%, gold and experience received by enemies upon killing you is reduced by 29% - 4200g

Reasoning: Might be a viable upgrade if your team is dying a lot, or if you have a bad player and you don't want him to feed the enemy too much.

Fort Upgrade: changed

Rank 1: Forts are upgraded with Trebuchets - 2500g (reduce war rank requirement to level 5)

Rank 2: *NEW* Forts gain 25% additional hp and they increase the armor of nearby allies by 15% in a radius of 15 yards. Trebuchets fire 20% faster. - 3500g (War rank 6)

Rank 3: *NEW* Forts gain 50% additional hp, and are upgraded with moats which decrease the movement speed of nearby enemies by 15% in a radius of 15 yards. Trebuchets fire 40% faster. - 4500g (War rank 8)

Reasoning: Right now forts are just a beefed up version of arrow towers. Ranks 2 and 3 would make them more of a strategic asset and add some interesting gameplay elements, especially in Fortress games.

Finger of God: changed

Rank 1: Does damage equal to 20% of the target's hitpoints - 4000g

Reasoning: Changed it to % from static damage so that it scales properly. 20% dmg is not that great, but it may kill a running demigod or change the tide of a battle. So war rank 10 and 4000g are fair I think.

Final notes:

My opinion is that, if a team invests in citadel upgrades, then buildings should become a significant threat to the enemy. I do not believe that the current situation, where towers are important only at the beginning, is undesirable because it leads to one-dimensional (i.e. demigod vs. demigod) gameplay. Fortresses, and to a lesser extent, towers, should be something that the enemy actually fears throughout the whole game, and creeps should be more than exp/gold fodder.