If you remove Bridge Middles it will change the appearance of the Tiles underneath to Water Shallow
If you remove Bridge Ends it will change the appearance of the Tiles underneath to Desert
If you remove all other Buildings with the exception of Walls and Docks it will give you Grass 3(I think), it is the Patchy Grass
Placing Bridges Gives you a shoreless rectangle of tiles, that means that only units can walk over it, no ships going on the edges. If you want to just change what the Tiles look like create then remove the bridges, this means you can have a flood spell if you have the arena in a desert.
I therefore can explain the Trigger Trick Invisible Farms, Ckn Chaos' technique basically creates another farm on top of the other one and then removes it changing the terrain underneath, you cannot select Grass 3 so it means you cannot select the Farm, Bridges can be used to make invisible Farms too. One bug is that you can select the Farm when It's burning, that's because you can select Fires
That is the theory behind Ckn Chaos' Invisible Farms. If anyone elsew has a Trigger Trick, I will try and guess the theory behind it.
Haven't been here in years