How do you get higher tier units?

[TidBuryFukles]

I am quite far in this game and still can only build tier 1 units. I am on my 3rd tome and haven't seen where tomes unlock them. Am I missing something (again)?

[BornToKill]

Pick tomes of higher tiers, they will get you better units.
Also improve your city to get access to your tier 3 units.

[AnemoneMeer]

There are two ways to unlock them.

Cultural Units are unlocked by upgrading your town hall building. As it levels, your city can sprawl out to larger sizes and build better units.

Magical Units are researched. Not all tomes have them, and you aren't always going to draw into them without some rerolling.

[~ Fabulous ~]

culture specific unit - from upgrading town hall.

affinity specific unit - from tome, if it say summon, you need to summon them, is it does not say anything you need to build them in your city.

Special units like fairy and dragons - you need to capture an ancient wonder and annex it to your realm, you then can recruit special unit through really the lieges