Games like Decimation
Games like Decimation include Age of Darkness: Final Stand, Aliens: Dark Descent, Creeper World 4, Company of Heroes 3, and Infested Planet. We matched them by the tags players give Decimation on Steam (Tower Defense, Action RTS, Sci-fi, Top-Down, and Real Time Tactics): closest matches first, and among equally close ones the better-known games.

Age of Darkness: Final Stand
Shared: Tower Defense · Action RTS · PvE

Aliens: Dark Descent
Shared: Sci-fi · Real Time Tactics · Strategy

Creeper World 4
Shared: Tower Defense · Real Time Tactics · RTS

Company of Heroes 3
Shared: Action RTS · Strategy · RTS

Infested Planet
Shared: Tower Defense · Top-Down · Sci-fi

Taur
Shared: Tower Defense · Sci-fi · Strategy

Broken Arrow
Shared: Action RTS · Real Time Tactics · Strategy

Door Kickers 2: Task Force North
Shared: Top-Down · Real Time Tactics · Strategy

Warhammer 40,000: Dawn of War - Definitive Edition
Shared: Sci-fi · RTS · Strategy

Creeper World 3: Arc Eternal
Shared: Tower Defense · Sci-fi · Strategy

DG2: Defense Grid 2
Shared: Tower Defense · Sci-fi · Strategy

Mud and Blood
Shared: Action RTS · Real Time Tactics · Top-Down

Artisan TD
Shared: Tower Defense · Difficult · Real Time Tactics

Bloons TD 6
Shared: Tower Defense · Strategy · 3D

Thronefall
Shared: Tower Defense · Action RTS · Strategy

Age of Mythology: Retold
Shared: RTS · 3D · Strategy

Anomaly Warzone Earth
Shared: Tower Defense · Sci-fi · Strategy

GemCraft - Chasing Shadows
Shared: Tower Defense · Top-Down · Strategy

GemCraft - Frostborn Wrath
Shared: Tower Defense · Top-Down · Difficult

FTL: Faster Than Light
Shared: Sci-fi · Strategy · Difficult

EverSiege: Untold Ages
Shared: Action RTS · Tower Defense · PvE

Axon TD: Uprising - Tower Defense
Shared: Tower Defense · Real Time Tactics · PvE

Outpost: Infinity Siege
Shared: Tower Defense · 3D · Sci-fi

Element TD 2 - Tower Defense
Shared: Tower Defense · Real Time Tactics · PvE