Heretic is one of my all time favorite games and it runs great on the C1000.
If you have already installed Doom as per the above instructions then just install these 2 files:
Heretic engine – heretic-engine_1.0.4-2_arm.ipk
Heretic game file – heretic-demo_1.2-3_arm.ipk

If you have not installed the Doom files, then you have to install the following:

libSDL-net_1.2.5cvs-1_arm.ipk
libSDL-mixer_1.2.5cvs-1_arm.ipk
libsdl_1.2.5-slzaurus20050731_arm.ipk