Fenridge would be good but I think not so slighted. Round Top has no info at all and that could be partially slighted but still functional - or modified from a ruin. Maybe that the actual tomb bit of Knights Tomb could be a ruin long since inhabited, away from the town / village, which contains lower levels that are more intact with the tomb in it. A bit like Sturm's in Dragon Lance. Maybe it has consecrated ground protecting it from evil like a paladins ability.

If you want a full on ruin then you can add an extra one almost anywhere since the points on the map were more for inhabited areas at distances that allow you to sleep secure and refuel. Anywhere off the beaten track as they say is no mans land. A bit like this 4e points of light thing.

Mere Fort is marked as a ruin from attacks from Lizard Men coming out of Death Chaser Fen. Might want to see Helium tho as he has taken an interest in that too.

My tile was supposed to be a generally dangerous area. The current set of roads represent the present state of it. Who knows how that might have been different in times past. Some castles might be half submerged by now in swamp. Roads might go nowhere or into lakes.