I am end of Ch15 in one city and early Amuni in the other. Both cities are in 10chest/wk FS's, so I have a steady supply of relics and spells. My choices now reflect the benefits I've received from playing the Spire for over a year. At one point, I had 3 full Spire Library sets out to build up my CC's (150+ in both cities now) and that also made my inventory of spell fragments insane (500K+ in both cities). In the Ch15 city I produce alternating MM & CC 24/7 and in the Amuni city I produce all CC's 24/7 along with playing the Spire to maintain those levels of inventory. What that means is that I have the ability to craft anything I want any time I see something I like.
Pet Food every recipe, every time I see it
Temporary Military Boost bldgs, every time I see one
Military unit-producing bldgs every time I see one
Time Boosters (though not the 5, 2, or 1hr ones unless there's absolutely nothing else; I win so many of these in the Spire)
Sometimes a supply instant if there's nothing else. I use those to replenish supplies when I'm pushing through troops with a fed Brown Bear, but I get many supply instants from the Spire, too.
^^All of those are to improve the military side of my city. I feed my Fire Phoenix several times/week to boost performance in tourneys and in the Spire. I also feed my Brown Bear once/week to increase troop production. I use the unit-producing bldgs to fill up the guest race space between chapters, or I nuke them when I need the room for lvl 1 bldgs for events/FA's. So, I like having a few in stock as replacements. The Spire has also given me a good inventory of teleport spells (90+, both cities), meaning I can move other buildings in and out of inventory as the mood strikes me as well and replace them with unit producing bldgs.
Festival Merchants (for the seeds; not every time I see one, though)
1x1 pop/culture bldgs (I did these at lower chapters, one RR/upgrade and it completes an event quest to 'upgrade a bldg') Unicorns work for this, too, but need 2 RR's.