Disclaimer: This is from personal experience
Time and time again I see people use the same siege in the wrong ways and wonder why either their offense failed or defense failed drastically and its because I noticed people using the wrong siege weapons for the wrong things.
So heres a small guide to teach you to become an engineer of cyro pvp.
Catapults. There are 3 types. Meatbag, Oil, and Scatter shot. They are short ranged offensive and defensive anti personal siege. I always see people spam meatbag and thinking its enough however each siege has its own use and you should diversify on how it should be used. All catapults are fast reloaded and short ranged.
Meatbag - meatbag is the most used one and is easily counterable with a simple purge, either from the support tree or from class abilities that get used a lot. Despite lowering heal meatbags aren't effective against large groups but smaller and individuals. Large groups will simply cleanse and move through it without slowing down. Meatbags are best not in the push but in the flanking areas or in smaller groups and offense. Don't use meatbags on large groups as they are generally ineffective.
Oil catapults. This is the area denial catapults. Doing flame damage upfront it puts avery strong reapply snare on the ground. Best used defensively on choke points this can and will stop large groups in their tracks. Even though its not used for damage its not easy to get rid of.
Scattershot catapults. This one is again very good for large groups as it does leave a short lived aoe which makes any player effected by this to take more damage. Great for both defense and offense. Great middleman catapult for both large and small groups.
Now for balistas. Balistas are anti siege and personale weapons that are great for both offense and defense but are weak to destroy structures. Having a medium reload time these are great as counter siege especially on defensive positions on either elevations or distances. However balistas do get misused in the event for offensive seiging and can take forever to get a keep flagged or a structure destroyed. However their counter siege and anti personale capabilities on a distance is great and is highly destructive.
first is the regular balistas. Does a small bit more to structures and heavy damage to players, this should always be aimed at enemy siege instead of a group of players as the siege will be destroyed quickly.
Fire balistas and cold fire. Does a ton of damage over time to players and cold fire amplifying it.
Lightning balistas. Snares enemies, does massive delayed damage, and is great against large groups. Best used defensively.
Now for trebuchets. Trebuchets are medium to long range destructive forces of nature that can even shoot over keep walls. Never place these on walls of a keep, you can use behind a wall of a keep and hit people over it. It is highly destructive on structures and should be used on walls and doors. While having a longer reload time these can easily be also aimed at enemy siege machines and quickly destroy them or enemies itself. They do deviating damage.
first is stone and cold stone trebuchets. These are the offense ines that should be used against keep walls. Just 10-15 of these can easily flag a keep quickly especially if all used at once. This will prevent enemies from porting over to it fast.
Fire and coldfire do heavy fire damage over time and just like the cold fire balistas do more damage than their fire counterpart.
Ice trebuchets do heavy damage as well as a snare.
Now for lancers. All lancers are multi shot defensive anti personale siege weapons. They do heavy damage and always put on their elemental effect.
Oil pots. An elevated anti siege and personale weapon causing massive damage in an area and area of denial.
Battering ram - requires 3-6 people and only damages doors.
There is a set in the game thats called siege masters while not best for front lines does allow you to see enemies even behind cover while using seige weapons and preventing you from being knocked off of siege.
Hope this helps and if you got any tips let me know.