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Buy the game on GOG and then install or open the keeperfx to play.

(install on GOG\dungeon keeper) (the installation path from GOG).

http://keeper.lubiki.pl/html/dk_keeperfx_dwnld.php
There's already a pinned thread on it: https://www.gog.com/forum/dungeon_keeper_series/keeperfx_hires_dk1_latest_builds_and_install_instructions