You need to build the bridge to get the creatures to the other side.
Build the bridge and press T to test it. Press escape to go back to building it. The bits the creature can walk along are build along the green line. Click on pieces to delete them (as long as you aren't able to place a piece there). Click on anchor points and press delete to delete those. Your bridge needs to be made out of triangles to be effective. Otherwise it will just break. Use the orange fixed anchor points to support your bridge.