Sandora and I made the same decision: we absolutely couldn't let demonized creatures approach the valley!
The base had just finished its basic construction, and although it had a certain defensive capability, it still seemed somewhat fragile in front of the hordes of demonized creatures that had been enhanced by the Abyss Power. Even though our various ultimate defense towers were impressively powerful, we couldn't use them unless absolutely necessary. Abyss Power wasn't foolish; although they did not have much self-awareness, they could be described as geniuses in battle. If we exposed our immature powers to the Abyss now, we would later have to face the enemy's frenzied onslaught, leaving us no time to freely develop and escalate our technological prowess... Ahem, I digress.
In short, our foundation was still weak and it wasn't yet time to go all out against the Abyss.