Percy jackson book question (BOTL)?

Alright, it's been a while since I read the fourth book so please tell me if I got something is wrong. Okay so, in the fourth book, Battle of the Labyrinth, the main reason the Titans wanted to find the Labyrinth was to infiltrate camp correct? But the barrier lets demigods in, and the Titans had demigods on their side. And you can invite monsters in as long as you had permission from someone in camp. So metaphorically, couldn't the demigods on the Titans side waltz right in and then invite all their monster buddies and save a lot of trouble. Whats was the need for the Labyrinth?