You need to have the entire area inside your walls covered with no-spawn items. These each make a 20m radius zone no enemies can spawn in. These include most interactive objects you can build, such as torches, workbenches, fires, furnaces, fermenters, beds, windmills and so forth.
This does not include simple structural objects like walls, roofs, floors, doors, and so forth.
If you don't cover everything enemies can freely spawn.
The best way to test is place things down so you have coverage, then go make loud noises on the other end of your base, then come back and see if anything showed up inside your base. If so you need to figure out where the gap in coverage is. Repeat.