using raise land will turn cliff tiles on the coast into beach tiles. the best result for creating a beach is when there are at least two passable land tiles adjacent to where you cast raise land. with only one adjacent land tile water tiles may be created instead of the intended beach tile. the raise land spell only costs 5 mana to cast so it usually takes very little time to fix such beach tile creation errors
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the practical use of transport ships is questionable as long raise land can create beach land bridges to forbidden or unreachable map areas and commanders can build roads to hasten movement
a transport ships moves 8 tiles, but this is quite slow in comparison to units moving at least 12 tiles on a land tile with roads (4x faster MP with roads, most units have 3 MP, so moving on roads is for 12 tiles with 3 MP)
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though not related to transport ships another use of beach tiles is to dam up a river that dumps into a lake or sea. this may cause one or more of your cities to get an error that claims it is no longer contiguous with capital city and hence suffers a new 15% city unrest penalty
I have found no general rule of what triggers this bug. in some cases after a few turns the bug disappears in one or more of the bugged cities (and hence the unjustified 15% city unrest penalty for those unbugged cities)
