Without the play/draw rule, a first turn Tower is all but guaranteed to give you a decent amount of life when you're running a deck without a steady curve and it was your best defense against the antithesis of the Tower, Black Vise
In their heyday, they inspired such unprecedented moves as the first balance-motivated ban in Vintage, a card being preemptively banned in Commander prior to release, and an entire mechanic being significantly reworked via official errata
Price: Often hundreds to thousands of dollars
The Locust God isn't terribly large, but it is very difficult to get rid of, which is almost as good
One-drops are super important alongside Merfolk Lords and Cenote Scout is a strong one to boot
Central to this is understanding how Pokmon cards function within battles