Fun, limited, analysis.
I say limited since GBs & Tribal squares and other buildings can create alternative strategies to acquire goods instead of just pure good buildings. Once unrefined versus refined enters the picture, I feel the 'square cost' comparison becomes even more murky.
I agree with the sentiment of 0% defense equaling more profitable/productive city. For my current situation, I'm debating on kicking out some RoF to free a slot for a Blacksmith to handle DCs. I'll probably keep my watchfires since they are 'rare' for me and it is easier to find a corner or end of a road for them. I've only been plundered a handful of times and to have had a defense that would of stood a chance to rebuke the pillager, I would have needed to sacrifice a T.Farms worth or more of space to dedicate to defense. Losing 5FP on a rare basis is nothing compared to the opportunity cost for 'saving' it.
To make the analysis better, we would need to take into account the total space investment needed to 'deter' attackers. How often does a 30% defense deter a 0% attacker? A 30% attacker? A 90% attacker? What is the optimal defense to shoot for to repulse 'most' raiders? How many raiders, on average, will breach & pillage you per week? Are we focusing on good buildings only, or should we have cities with Only goods, Set buildings & Goods, T.Farms & goods, Only set buildings & T.farms, etc.? Aid is being tracked, should we track if the player is also a pillager and if that effects the amount of attacks on a city and pillages? I'm sure we could brainstorm even more confounding variables.
That's why I like FoE so much. There are enough variables to construct multiple ways to deal with being pillaged. Stack Watchfires/RoF/2GBs to dissuade everyone except the most robust attackers. Don't place buildings that can be plundered. Collect on time. Plunder, be plundered and only care if the rum is gone. Place valuable mutli-production buildings and only use the 5 min setting... Disconnect the road or building from the town hall...? Delete everything and send a road message to those vile pillagers!
That all being said...
I may scour my event log and attempt to contribute some data into the thread instead of my usual opinion screed.