Games like Round 99
Games like Round 99 include Cuphead, Enter the Gungeon, Bit Blaster XL, Spelunky 2, and Rise & Shine. We matched them by the tags players give Round 99 on Steam (2D Platformer, Shoot 'Em Up, Bullet Hell, Difficult, and Twin Stick Shooter): closest matches first, and among equally close ones the better-known games.
Cuphead
Shared: Difficult · Platformer · Local Co-Op
Enter the Gungeon
Shared: Bullet Hell · Difficult · Twin Stick Shooter
Bit Blaster XL
Shared: Shoot 'Em Up · Bullet Hell · Difficult
Spelunky 2
Shared: 2D Platformer · Difficult · Platformer
Rise & Shine
Shared: 2D Platformer · Bullet Hell · Shoot 'Em Up
Pizza Tower
Shared: 2D Platformer · Platformer · Cartoony
Celeste
Shared: Difficult · 2D Platformer · Platformer
Super Meat Boy
Shared: 2D Platformer · Difficult · Platformer
Voidigo
Shared: Twin Stick Shooter · Bullet Hell · Shoot 'Em Up
Geometry Dash
Shared: Difficult · 2D · Platformer
Nex Machina
Shared: Bullet Hell · Twin Stick Shooter · Shoot 'Em Up
Beat Hazard
Shared: Bullet Hell · Shoot 'Em Up · Twin Stick Shooter
Chippy
Shared: Bullet Hell · Shoot 'Em Up · Twin Stick Shooter
Ikaruga
Shared: Shoot 'Em Up · Bullet Hell · Difficult
Super Time Force Ultra
Shared: 2D Platformer · Bullet Hell · Shoot 'Em Up
Touhou Tenkuushou ~ Hidden Star in Four Seasons.
Shared: Bullet Hell · Shoot 'Em Up · Difficult
Geometry Wars™ 3: Dimensions Evolved
Shared: Shoot 'Em Up · Twin Stick Shooter · Bullet Hell
Touhou Koumakyou: New Classic - the Embodiment of Scarlet Devil
Shared: Shoot 'Em Up · Bullet Hell · Difficult
Assault Android Cactus+
Shared: Bullet Hell · Shoot 'Em Up · Twin Stick Shooter
Hollow Knight
Shared: Difficult · Platformer · 2D
Warhammer 40,000: Shootas, Blood & Teef
Shared: 2D Platformer · Shoot 'Em Up · Bullet Hell
Katana ZERO
Shared: 2D Platformer · Platformer · Action
Bread & Fred
Shared: 2D Platformer · Difficult · Local Co-Op
SANABI
Shared: 2D Platformer · Difficult · Action