Once I rounded out the initial cast of enemies in Reginald, I started to realize that the game lacked a slow yet tough character for the player to grapple with. So about a month ago I designed a knight character and worked out all (or, most) of the kinks. The knight has much more health than other enemies, its attacks deal damage almost equal to 100% of the player's health, bullets bounce off the armor (and back at the player) and grenades, while still doing damage, do not explode on impact as they do with other enemies.
Completed, I put the knight down and worked on other areas of the game, a couple of levels and random polish/improvements, as I was not yet to the actual level that would introduce the enemy. I recently started designing the level, and immediately was faced with an issue: the player can simply jump over the knights, avoiding them entirely. What's the reason to engage them?
The player can generally avoid other enemies, but as some of them fire bullets or shoot grenades they present a more kinetic challenge. Also, I wanted there to be motivation to take down the knights, as they are challenging enemies and fun (I think) to engage. I upped the coins that the knights drop, but I felt this wasn't enough motivation.
Concurrently, very near the level that introduces the knight there is a cafe where the player can purchase and consume a couple of items that grant temporary fire or ice vomit. Fire being a mechanic that has been introduced already, yet ice a mechanic that I was contemplating how to introduce within the next few levels. These mechanics let Reginald attack enemies in different ways, but also open up new paths inside levels and the hub world. What was the best way to introduce the ice mechanic, ideally before the next area of the game (which will rely heavily upon it)?
I realized I could combine these two design considerations together by creating two different knights - fire and ice knights. When defeated, Reginald can consume the heart of either for fire or ice vomit, and combined with restrictive level design, I suddenly had my reason to engage the knights. In a sense, they are items. In the level that introduces them, one recipe page (the "collectible" of the game) is past a large fire, and the other requires burning down a bridge to access. As a bonus, this introduces the ice mechanic organically and lets the player discover its usefulness without using any sort of tutorial.
Boom! Game design! Or something close, at least.