Final answer:
Prince Prospero attempted to avoid the Red Death plague by sealing himself and his friends inside an abbey, isolating themselves from the outside world. Despite their efforts, the plague found a way inside, highlighting the theme of the inexorable nature of death.
Step-by-step explanation:
The security measure taken by Prince Prospero to prevent the Red Death from entering the abbey was to seal himself and his friends inside, essentially creating a quarantine. They welded the doors shut, thereby cutting off any possibility of contact with the outside world where the plague was raging.
Prince Prospero believed that the stronghold of the abbey, along with their method of sealing themselves inside, would protect them from the plague's reach. However, despite their precautions, the Red Death found its way inside, which underlines a theme of the story: no fortifications can completely guard against the inevitability of death.