 The limited availability of fixed points may require the player to construct self-supporting bridges.These bridges must be able to withstand their own weight, as well as the trucks and any force they impart when landing on the bridge.Elements from "Portal" require the player to guide the trucks across the bridges, navigate through portals, cross over buttons to open doors or other actions, avoid coming in sight of sentry turrets or in contact with deadly fluid or laser fields, and use other "Portal" gameplay elements like propulsion and repulsion gels and launching pads.