Games like HarmonyTD
Games like HarmonyTD include Bloons TD 6, Orcs Must Die! 2, Slay the Spire 2, ShellShock Live, and Castle Story. We matched them by the tags players give HarmonyTD on Steam (Early Access, Strategy, Indie, Tower Defense, and Multiplayer): closest matches first, and among equally close ones the better-known games.

Bloons TD 6
Shared: Strategy · Tower Defense · Multiplayer

Orcs Must Die! 2
Shared: Tower Defense · Strategy · Indie

Slay the Spire 2
Shared: Strategy · Multiplayer · Indie

ShellShock Live
Shared: Strategy · Multiplayer · Early Access

Castle Story
Shared: Strategy · Multiplayer · Indie

Legion TD 2 - Multiplayer Tower Defense
Shared: Tower Defense · Strategy · Multiplayer

Plague Inc: Evolved
Shared: Strategy · Indie · Multiplayer

Dungeon Defenders
Shared: Multiplayer · Tower Defense · Strategy

Town of Salem
Shared: Strategy · Multiplayer · Indie

Warstone TD
Shared: Strategy · Tower Defense · Indie

Orcs Must Die! 3
Shared: Strategy · Tower Defense · Multiplayer

Buckshot Roulette
Shared: Multiplayer · Strategy · Indie

Sanctum 2
Shared: Tower Defense · Strategy · Indie

Rock of Ages
Shared: Strategy · Indie · Tower Defense

Mount & Blade II: Bannerlord
Shared: Strategy · Multiplayer · Indie

EmergeNYC
Shared: Early Access · Multiplayer · Strategy

Plants vs. Zombies GOTY Edition
Shared: Strategy · Tower Defense · Indie

Rock of Ages 2: Bigger & Boulder™
Shared: Strategy · Indie · Multiplayer

Tower Wars
Shared: Strategy · Tower Defense · Indie

Bloons TD 5
Shared: Strategy · Tower Defense · Multiplayer

Orcs Must Die!
Shared: Strategy · Tower Defense · Indie

Who's Your Daddy?!
Shared: Multiplayer · Strategy · Indie

Sanctum
Shared: Strategy · Tower Defense · Indie

SpyParty
Shared: Strategy · Multiplayer · Early Access