They sealed off the entire house with webs so that exit and entry had become forbidden. Anyone who stepped into the webs would fall under the control of the Marionette divine ability.
Even if the webs can't catch and control the person who wandered into it, they can form a blockade that will hinder movement and the web can still inform Legion of movement in the region so that they won't be caught unaware.
This didn't mean that the demigod was trapped, though. After all, there could be secret underground passages that led out of the compound that Legion wasn't aware of. But this was the best that they could do with what they had.
The demigod's house was also protected by arrays. So Legion couldn't enter the house directly to attack the demigod.
What they did to solve this problem was to sneak into the house. It was easy to do thanks to their current hidden state.
