Time for a refresh on all the new things we could add here?
I don't think it's so limited anymore to need a comprehensive list. Many event buildings now offer defense for attacking army, but sentinel outpost still does so most efficiently (in practical terms though its competition is likely the "blended" attack & defense buildings because while their defense efficiency is still pitiful compared to sentinel outposts you can view them as replacing other "pure attack" buildings that aren't in most cases much (if any) better than the "blended" buildings at attack for attacking army as well as "pure defense" buildings *other* than sentinel outposts).
The practical list (in my personal order of preference)
1) GBs
2) Sentinel Outpost - the only long-term good way to bias more towards defense if that's what you're aiming for. if you want more balanced stats though, lines 4-6 provide some excellent starting points.
3) Sun Temple - strong economy building that happens to offer pretty good defense for your attacking army as well
4) Nutcracker Guardhouse & (st-pats-beta-spoiler) - the most efficient tile-able hybrid attack/defense/economy buildings atm are spawns from other buildings
5) Chocolate Factory & (st-pats-beta-spoiler) for making the previous line
They're not bad in their own right, and would slot into one of the next two lines otherwise.
6) Eagle Mountain, Heroes Tavern, House of the Wolf - for providing a decent side of defense (almost as good as anything not already listed above) while being competitive with "pure" options for providing attack & economy stats.
7) Pergola, some Fall Fields layouts, some Butterfly Fields Layouts - same as 6, but not as much defense to go with the attack
8) Fiore Village, Olympic Treasury, Ancient Obelisk, et al - Only Defense + Economy hybrids, too many to bother listing them all. Some are better than others, but mostly at the economy half - they're mostly pretty tightly clustered on how much defense/tile they add. Flower Maidens (a one-time premium offering) might be the only one that adds significantly more defense in this category. But it's still outclassed by other options in the previous 7 lines imo.
9) Roussoi Track, Gate of the Sun God Level 2, and a few other chain buildings - despite using space purely on defense these are *not* efficient at all - most of them other than the roussoi not even being competitive with an economy/defense hybrid at defense alone! But if you need defense in a hurry at the start of a new city for one reason or another (such as fighting way ahead on continent map), it might be more reasonable to pick up some of this stuff in bulk than wait for good stuff. And delete it later when you no longer need it.