City defences are now more weaker with this change. The change most people wanted to keep plunderers off their doorsteps was increased (+) attack power for defending armies. This change put the whole balance of the game in jeapordy, possibly disregarding GB's since they are now ruled obsolete.
I think you are mistaken (and so is a few others) if you think that taking DEF for defending army away from OT weakens city defenses. DEF for DEFENDING army is virtually UNLIMITED in this game, if you do play expeditions, I have so many ritual flames that If I try to build all of them, I'd fill up half of the city. Plus watchfires. And these buildings are unlimited from GE. While the new sentinels are available just here and we can only speculate IF they gonna be somewhere else in the future, but we can always speculate about any future. As for this very moment, an event building with DEF for defending army is pretty much useless, period. The OT is 4x5=20 squares. That is 10 ritual flames = 60% def. or 20 lvl 2 watchfires, that's 120%. Placing a building with 20% def of defending is just ridiculous.
yes, same could be said for the (size of) sentinels with one little difference. Nobody could have them until now and hard as you try, you cannot get 76 in this one event... (my actual current inventory of ritual flames on live server) unless you are very rich and spend hundreds of dollars.
That is the reason why I believe that it was CORRECT move to change it for DEF for attacking army, which you cannot get anywhere else than here and now and in very limited amount.
As for rendering GB obsolete - I don't think so.. Zeus statue is only 6 squares, at lvl 10 it is 30% which is more than 6 sentinels on lvl 2, so why would anyone think it is obsolete is beyond me. Not counting the fact that you can keep leveling it and keep getting bigger bonus...
Castel del Monte is worse regarding the size, but gives FPs as bonus. I just don't see any sane player trying to demolish it for any other building... and so on.