I'm trying to launch the game on Linux Mint and also get a silent failure. Starting the game from the command line gives me this:

ExecSteamURL: "steam://rungameid/218410"
Game update: AppID 218410 "Defender's Quest: Valley of the Forgotten", ProcID 5356, IP 0.0.0.0:0
ERROR: ld.so: object '/home/[...]/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/[...]/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
ERROR: ld.so: object '/home/[...]/.local/share/Steam/ubuntu12_32/gameoverlayrenderer.so' from LD_PRELOAD cannot be preloaded (wrong ELF class: ELFCLASS32): ignored.
x-terminal-emulator: symbol lookup error: /usr/lib/x86_64-linux-gnu/libatspi.so.0: undefined symbol: g_type_class_adjust_private_offset
Game removed: AppID 218410 "Defender's Quest: Valley of the Forgotten", ProcID 5356

Any ideas?

I was able to launch it by running in the game directory. ("adl" is a part of Adobe AIR installation.) Beware that initial launch may take quite a while (e.g. a minute) without anything happening, apart from "adl" continuing to run.

If this works, set the following as the custom command to launch it from Steam:. Custom command is at "right click on the title in the game list" > "Properties" > "General" > "Set launch options...".

There is another way (based on ArchLinux wiki entry[wiki.archlinux.org]):

1. Make sure "/opt/Adobe AIR/Versions/1.0/libCore.so" exists (by installing Adobe AIR and symlinking as necessary).
2. Run

You should try again now that the HD remake is available; it doesn’t require AIR anymore.

Yep. So glad about the update. I can finally play again!