Well I mean there are two separate issues : what you CAN get, and what you would LIKE to get.
Using subpar defensive options because it's hard to get as many sentinel outposts as you can get is definitely somewhat a thing still for some - part of the issue with a 2 level 1x1 is it takes a lot of wins of specials to make a dent even if it does outclass everything in terms of defense efficiency.
But in that situation it really isn't a "tree of vitality" vs "sentinel outpost" situation - it's a you-want-both situation
Replace some of your attack-only options with tree of vitality, keep up on that front, and get higher defense as a side while you continue to try to build more sentinel outposts as you get the chance as well.
But even if I could have a city where every single tile other than a GB was a level 2 sentinel outpost, that isn't the city I would want anyways.
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My cities that want equal-or-higher-defense have largely caught up now with similar tradeoffs.
My SAJM diamond mine(ex-main) is sitting at 802/800; it canned most of its pure attack for a mix of more wells and overall-more-efficient attack/defense hybrids (including what eagle reserves I could win for free from wildlife events). The next step there will depend on what the requirements for GE5 will be (and whether I decide it should do it) - as I have more than enough boost for GE4 atm and that's all I really *need* there for now.
My VF main is sitting at 1520/1446. It removed a lot of one-dimensional attack buildings in favor of eagle reserves during last wildlife, paying some diamonds to do so (but it was relatively diamond-efficient as far as winning extra buildings go). It has 18 eagle reserves in total. Attack is still ahead on it because it also has 42 wheat fields in fall fields layouts that I consider too good to give up for the foreseeable future - but they are the only attack-only buildings I intend to keep longterm still at this time. I hope to have Druid Huts + Nutcrackers + Trees of Vitality replace some of what remains for one-dimensional buildings (attack-only, defense-only, *and* "nice BG collection") and eventually get my defense *higher* than my attack there.
Increasing good hybrid-options can only help your quest to get your defense closer to your attack *as well as* continuing to build sentinel outposts until such time that you feel you have "too much defense".
The buildings with "least % wasted military squares" as it stands now (wasted military squares being defined as how much space you could save by going with the best-one-dimensional-option for attack (WP Lv 2) and defense (sentinel outpost) instead):
1) GBs & Yggdrassil - -100% or better - but you can only have 1 of each
2) Tree of Vitality - -27%
3) House of the Wolf - -6%
4) Nutcracker Guardhouse - -5%
5) Fall Fields 13 pc attack (7 wheat, 2 ochre, 2 begonia, 2 scarecrow) - -1% (note that this is just my preferred layout; there's an even-more-military 13 piece that should rate a little higher but give less FP/goods and have a few more awkward gaps to fill (with sentinel outposts probably)
6) Sentinel Outpost, Winner's Plaza Lv 2, Eagle Reserve, Archdruid's Hut - 0%
7) Heroes Tavern - 3%
8) Tree of Silence - 7%
9) Pergola - 9%
This is specific to SAJM. In ages below AF, Fall Fields becomes the standard for attack on top of providing a little extra defense and is pretty much unbeatable still (augmented with sentinel outposts if desired). Tree of Vitality needs AF+ and Nutcracker Guardhouse needs OF+ or they're significantly worse. Other buildings that scale with age similarly suffer.