Quake 1 and 2: Get the patches that match Steam version, and try unpacking them to get exes and DLLs.

Heretic and Hexen: If CD games don't check protections, maybe using a patch would work.

Half-Life and expansions: I think that data files from CD and Steam are different, so they won't work with standalone exes.

Return to castle... checks for CD, so even if data files are equal and you get a non-steam exe you'll run into troubles.

Heretic and Hexen both have some kind of source ports, don't they? You could use those instead. Same goes for Quake and Quake II, I suppose.

In the case of Quake you can install the shareware version and copy only PAK1.PAK over to get the full original thing. Still the issue of no CD music though 😜

Jorpho wrote:

Heretic and Hexen both have some kind of source ports, don't they? You could use those instead. Same goes for Quake and Quake II, I suppose.

Yes, but source ports may not be as close as original and even source code releases may not be exactly the same as published code (Quake 3 had some licensing problems and audio system is different). Other source ports may need more CPU/GPU/RAM than original games, so I'd try to make the original code work.

Some time ago, I put the Quake DOS exes in the Steam folder and they worked in DOSBox but they lacked music (because Steam version has no soundtrack). As you said, the datafiles of most steam games works with source ports (I used Quake 1 and 3 data ina Raspberry Pi) but there are some exceptions. When Steam released some Lucasarts adventures, the people behind scummvm choose to not support those releases (I don't know if that releases are supported now). Although first releases of Half-Life were the same as the CD ones, they stopped publishing patches for CD versions (so Steam data was more advanced and, I suppose, data files were incompatible with older executables); some time later they changed the data format (to pack it more?) so definitely datas from Half-Life are incompatible.

Zup wrote:

When Steam released some Lucasarts adventures, the people behind scummvm choose to not support those releases (I don't know if that releases are supported now).

FYI, there is a tool available that will make the Steam releases compatible with ScummVM, but its existence is not widely publicized.