one of the choices for a lev 3 conclave is oracle which gives +1 city essence. this is a COMMON choice
there is a RARE choice called prophet which gives the more desirable +2 city essence
I tried going back to a previous save to get a RARE city lev choice. it did not work. not patience enough to try this ten times in a row
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the city lev upgrade choices are listed at
https://forums.elementalgame.com/437964/page/1/#3294445
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another way to boost city essence is with the buildings scrying pool (with enchanters faction trait) and ritual site (available to all factions). each of these bldgs gives +1 city essence
finally the last way I know of to boost city essence is to install a mod that adds an upgrade to ritual site called mythic ritual site. when built this city improvement replaces the ritual site to add +1 city ess (+1 ess goes to +2 ess). it is a faction unique bldg so it can only be built in one of your cities
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original post updated to my current knowledge of the game -
I just noticed while playing as Lord Relias of Altar in the Anthys (2nd and final) scenario that at some point my oracle upgrade morphed into a prophet upgrade that gives +2 city essence. it turns outs the oracle upgrade did not upgrade. rather that for one lev 3 CONCLAVE instead of the COMMON choice of oracle (+1 ess) I received the RARE choice of prophet (+2 ess). since I was unaware that there were more than the listed three choices for city upgrade I made this choice not realized that oracle was NOT one of the three choices
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on the general topic of city building strategy see my other post in this forum
https://forums.elementalgame.com/531170/page/1/#3952570
