TR1 CUSTOM LEVEL GENERAL INFORMATION:

A) As a rule of thumb, the closer your system is to the original system the game was supposed to be played, the less problems you will encounter with playing the TR1 custom levels.
B) Though not required in most cases, if you experience some issues running TR1 custom levels like not getting any sound effects (gunfire, footsteps, etc.) it is worth to install (or reinstall) the original game for reference, though doing so will usually solve the issues by itself.

TR1 CUSTOM LEVEL SPECIFIC INFORMATION:

1) Playing on Systems from Vista onwards:
When trying to play on Windows systems from Vista onwards (Vista, 7), you will most likely get a "This system does not support fullscreen mode" error message. It is still possible to play the levels on these systems, but you have to use additional programs such as DosBox to get the levels to run. Please refer to the Vista7installation.txt for further information.
2) Hearing Background Audio:
Apparently, same as for TR2 custom levels, you can hear background audio tracks if you insert the original TR1 CD in your CD-ROM. From experience it hasn't been as simple as that (when tested on XP, Windows7), but it might be related to point A)
3) Taking Screenshots:
It is not possible to take screenshots without the help of additional programs in DOS games from Windows NT based systems (NT, 2000, XP, Vista, 7) onwards. You can, however, if you use DOSBox and PrintScreen key whilst playing. 