Defensive Strategy

the purpose of caltrops is to slow the attackers at maximum range while the army and other defenses attack them at range
caltrops do very little damage, they rarely kill troops alone
caltrops are completely ignored by cavalry, supply wagons, and Siege Engines, they move right past.
caltrops should be used with archers and other range troops defending your city, or with wall-mounted crossbows
Spiked Barriers
spiked barriers stop both types of cavalry, Siege Engines, and supply wagons until they are destroyed
spiked barriers do some damage - they can easily wipeout supply wagons if they are ignored
spiked barriers are completely ignored by infantry troops - they walk right past
like caltrops, spiked barriers work best when holding enemy attackers at range for your other troops and defenses
traps are single-use defenses - they will be destroyed when they attack enemy troops
traps do a lot of damage, nearly guaranteeing that the attackers will take some losses
traps may be bugged - it is easy for a small group of fast troops to take out a large number of traps in a single attack
Wall-Mounted Crossbows
crossbows are very effective defensive units - they do much more damage than archers, and they have a longer range
crossbows work very well with caltrops and spiked barriers to hold the enemy at their long range
crossbows have shorter range than ballista and catapults, so they are suceptible to attacks from these units
Defensive Trebuchets
trebuchets will only attack Siege Engines
trebuchets will completely ignore all other units, including supply wagons
trebuchets have an enormous range and very high damage, virtually guaranteeing that it will destroy an enemy siege engine before it can get into range
trebuchets are single-use defenses - they will be destroyed when they fire upon an enemy siege engine