Post-match analysis.

You can upgrade elves. Unicorns in the castle stand anyway.

They only stand if the centaurs aren't upgraded, but choosing which units to upgrade is actually a very controversial question, and depends in particular on the number of units you have. Personally, for example, I prefer 1-6 in the Citadel with both upgraded, which gives a good tempo at the start of the second week (it's much easier to buy the growth and faster to upgrade)