Games like Mech Fortress
Games like Mech Fortress include Mechabellum, Bloons TD 6, Legion TD 2 - Multiplayer Tower Defense, Creeper World 3: Arc Eternal, and Wall World. We matched them by the tags players give Mech Fortress on Steam (Tower Defense, Auto Battler, Bullet Heaven, Strategy, and Mechs): closest matches first, and among equally close ones the better-known games.

Mechabellum
Shared: Auto Battler · Strategy · Mechs

Bloons TD 6
Shared: Tower Defense · Strategy · Singleplayer

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

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

Wall World
Shared: Tower Defense · Strategy · Pixel Graphics

Warpips
Shared: Tower Defense · Strategy · Auto Battler

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

Heretic's Fork
Shared: Tower Defense · Bullet Heaven · Strategy

Dome Keeper
Shared: Tower Defense · Sci-fi · Singleplayer

Into the Breach
Shared: Strategy · Mechs · Pixel Graphics

BattleTech
Shared: Strategy · Mechs · Sci-fi

Dungeon of the ENDLESS™
Shared: Tower Defense · Strategy · Sci-fi

Infested Planet
Shared: Tower Defense · Strategy · Top-Down

Defense Grid: The Awakening
Shared: Tower Defense · Strategy · Sci-fi

Necrosmith
Shared: Tower Defense · Strategy · Top-Down

Dungeon Warfare
Shared: Tower Defense · Strategy · Pixel Graphics

Titanfall® 2
Shared: Mechs · Sci-fi · Singleplayer

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

Wireworks
Shared: Tower Defense · Auto Battler · Strategy

Sanctum 2
Shared: Tower Defense · Strategy · Sci-fi

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

Thronefall
Shared: Tower Defense · Strategy · Auto Battler

Revenge of the Titans
Shared: Tower Defense · Strategy · Sci-fi

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