It's best to watch the tutorial when you first create a monster spawn tower. My guess is: 1. There must be running water in the monster spawn tower, and there must be somewhere to stand, because monsters don't spawn in water. 2. Your mode is incorrect and should be set to hard mode. Peaceful mode is okay. 3. The monster spawn tower you built is not very effective. It is better to build more floors.
100% handmade, if you like it, adopt it!