Rotate Your Guardians
Strongholds of villainy hire monsters to protect areas, often leaving them in place for years or even decades at a time. While effective in achieving high levels of security, improving the local creature job markets, and aesthetic excellence, it should be noted that your monsters are only as strong as their training.
Guardians should be rotated every 3-5 years. Shift your deep-seated creatures up towards the front lines so they can stretch their abilities, pull your primary troops back so they may rest and enjoy the comforts of being closer to your dungeon heart.
If you can afford it, offer each monster patrol a year off between rotations. Let them explore the Netherworld — to gain experience, enjoy their hellish existence, and take time to simply enjoy themselves terrifying the landscape.