While not a "level," there is a famous farming strategy often referred to as a secret spot by the community.
: Most secrets are hidden behind walls with visible cracks. Shooting these will open new rooms. Investigate Dark Corners alien shooter 2 reloaded all secret place
Many secret areas are hidden behind walls that look solid but can be destroyed with weapons. Always shoot at suspicious-looking wall sections or corners. While not a "level," there is a famous