What's the max height something can fall from and not die? I want to have a bridge that I can drop out from under besieging goblins and splatter them on the rocks below.
A fall of 12 Z-levels should be fatal to anything. A fall of 8 Z-levels will kill almost anything, but may leave the occasional mangled survivor.
Two horses I have are starving to death in a grassy pasture. What do they eat?
Horses eat grass. Your pasture may not have enough grass remaining for them.
Is there anything that lets items through but not water? This is to be placed at the bottom of a drowning pit.
No. Floor grates will let water through but not objects, but there's nothing that will magically let objects through but not water.
For defensive purposes, what type of bridge is better in terms of pulling it out from under enemies? It seems a retracting rather than raising bridge, since that one doesn't fling them to potential safety.
Retracting bridges throw creatures up to 3 tiles away. Raising bridges throw creatures up to 11 tiles away. Either can throw a creature to safety, but raising bridges throw further. You can help prevent creatures from being thrown to safety by surrounding the bridge with walls.
Retracting floor grates and floor hatches don't throw creatures at all, but each one only covers a single tile, and they're vulnerable to building-destroyers.