This has just occured 10 minutes ago. I was playing on a custom battle (my unit of greek archers vs. a unit of Spartans) and my archers glitched into the battlefield floor. They continue to shoot their arrows out of the ground killing people. How can I avoid this in the future in order to have good "clean fights".
You can try reinstalling the game, but before you do that backup your save files. Alternatively, you can update the game and patch it. There are different patches and update but you have to install them sequentially to prevent more errors.
First you download the 1.3 patch which you can find here:
Then install the 1.5 patch which you can find here:
This should solve your problem but if you have RTW: Barbarian Invasion v1.3, you can upgrade to 1.6: