What do you think makes for good enemy placement?
There’s a lot of factors that go into this. In abstract, it’s about placing the enemies in spots where it is tough for the player to deal with them, but where the player has a variety of options for dealing with them, without allowing the player an easy way out.
This means also building the level around what the enemy can do. If the enemy is good at pushing, put them next to a cliff. If they toss projectiles that drop down, put them up high. If they require the player to jump, put down something dangerous to land on.
Castlevania 3 (and the Souls series) is my standard reference point for good enemy placement. Enemies are placed strategically, in places where you need to expose yourself in order to attack them, or manage some aspect of navigating the environment at the same time. Enemies are paired to create a combined threat, by each covering the spaces the other one doesn’t threaten, then they act at asynchronous intervals to vary the patterns that come your way, requiring you to solve situations on the fly, rather than just memorizing a solution.
Probably the most simple example of using enemies together is pairing a ranged enemy with a melee enemy. Castlevania 1 also has the classic example of the axe throwing knight paired with a medusa head.
Castlevania will force you to walk up stairs into bone pillars or patrolling enemies. It will force you to drop down onto crumbling floor to chase an enemy that moves forwards and backwards relative to you. Or force you to jump a gap onto crumbling floor as a floating enemy comes down from above. It will place bone throwing enemies up on high platforms or ask you to jump up into the path of axe throwing enemies. Dark Souls does a lot of the same.
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