I agree with everyone else in that it's probably just too dark, 3 oil lamps doesn't seem like enough for such a large area. But to be sure you can check by going into: settings - interface - turn on developer mode and turn on extended debug info. Then press ctrl f3 and on the right of your screen you can see the light values.
The numbers allll the way to the right in this image are the light values, they need to be 8 or higher to prevent monster spawning
We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.
