The Ceras Lake Ruins were a Sindar made Sluice Gate used to control the water levels in Ceras Lake. Inside the ruins is large room with a waterfall blocking the way forward and two maze like corridors on either side. After transversing the maze and stepping on three special switches that shut off the Water fall Freyjadour Falenas and his team were able to reach further into the ruins. Near the end the ruins they came across a pair of Diamond Suns and had to defeat them. Afterwards they found the control device used to open and close the sluice gates effectivly raising or lowering the water level in Ceras lake. Upon releasing all the water from the Lake Hatred Fortress was destroyed by a tidal wave. Later the Prince used the ruins to flood out the Godwin/Armes armies that were occupying the Castle.
It is possible that the Diamond Suns were a trap left by the Sindar Civilization - seeing that all Sindar Ruins seem to have Bosses in them.