The thing about the SPTs is that they are big, damned big. The holovids don't show you that, they just show them looking like the rest of the armored vehicles. Sure, they only come up to the hip of the average mecha, but that doesn't take into account that the beasts are 4 times heavier, and have way more firepower.
The Self Propelled Turret
The SPT is a ponderous slow vehicle that appears in the upper echelons of armor development. The main battle tank has one cannon, the Destroyer Pattern Tank has two, and the SPT has three. The defining characteristics of the SPT are heavy armor, a multi-tread caterpillar propulsion system, and a turret mounting either a large number of conventional heavy weapon systems, or a smaller number of naval grade weapon systems.
The Goliath SPT is a convention vehicle deployed by the Atlantic Federation. It is a heavily armored crawler that is armed with three 155mm long barrel cannons. These guns have long range, good accuracy, the ability to fire multiple types of rounds, and pack a heavy punch. The Goliath is an antique, mostly found in secondary and tertiary garrison forces, and when it isn't on active duty, it is linked into the power grid recharging it's 6 ton battery pack. The Goliath typically works as a mobile fire support base, or as an anchor in a defensive line. With less than cutting edge electronics, the SPT is less vulnerable to electronics warfare that is heavily favored by Amerikka Command
Typical Lay Out
The Typical SPT is as mentioned, a superheavy or ultraheavy vehicle with massive amounts of armor. The vehicle has a central power system, which feeds juice to electric drive motors in up to 8 sets of tank treads. These vehicles are not fast, and most advance at typical infantry overland speed. The hull will have numerous anti-power armor and anti-infantry weapon systems, mostly as deterrent to keep the vehicle safe from sappers, commandos, and armor-busting units that try to get close. The main weapons are housed in a large turret, and these weapons typically have a large degree of available movement. By allowing a high arc on guns, the mobile turrets are able to engage things taller than them up close (mecha) or function as artillery support. Liekwise, some very high end SPTs can use advanced targeting systems and rail guns to engage targets in low orbit.
Conventional Cannons are common, as they are reliable, relatively inexpensive, and can fire a variety of different types of rounds. High explosive rounds work for artillery work, while armor defeating rounds are good for taking out armored vehicles and mecha, and there are marker rounds, smart rounds, and a variety of arcanotech rounds such as freezer shells, thunderbolt rounds, and the like.
Magnetic and Rail guns are also common, as their high velocity and damage curve make them valuable for anti-mecha operations. While not capable of firing special munitions, most magnetic and rail systems have much higher velocities, and are accurate at much longer range.
Missile Systems are uncommon. Most fixed installations and defensive positions will affix missile launchers on other objects, so most SPT missile batteries are specialists, firing long range cruise missiles, drone missiles, or surface to orbit missiles. Given the range of the missile in the Cosmic Era, a SPT with long range missiles is shooting at things on the other side of the continent. As such, most missile battery vehicles are deployed with an eye towards anti-ship warfare, shooting down the aerial destroyers and frigates that make up the vanguards of hostile fleet groups.
Energy Weapon SPTs are fairly common, despite the often high cost of the systems. A rolling fortress armed with naval grade particle cannons or anti-ship lasers is a powerful weapon. Their massive use of armor ensures that the SPT will survive several major hits before going out of action, meaning that the tank can 'Light up the Sky' several times before going out. These SPTs are typically deployed in contested areas, where their intimidation factor can deter an assault. A tank armed with naval grade cannons and naval grade armor is going to trash a lot of expensive mechs before it goes down.
The SPT is a relic of the Second Dark Age when the mode of warfare dictated that unit longevity and firepower determined who would win the day. While resources were scant, a small number of large powerful vehicles could hold the battlefield. The SPTs and other now antique armor systems were the main weapons coming out of the Resource Wars. The mech assumed it's place as the new ideal balance of armor, speed, and firepower and the tank was relegated to the status of pariah. The SPTs avoided this fate as those that existed at the time were very much defensive weapon systems, and had only seen action in the positive role of defending cities and strategic points from hostile forces.
The physical resemblances between conventional tanks and the SPTs were minor enough that the vehicle avoided the backlash against the tank. They were larger, much slower, and their service record was dramatically different. Through the second dark age, the industrial powers defended themselves with a mixture of power armor and early mecha, but the core of the defensive line was the SPT, and men and women manning defensive cannons, walls, and other implements of warfare not seen since the first dark age and WWI.
The SPTs were in prototype stage development in the Petroleum Era, and a handful were built and used in the Resource Wars. These vehicles were seen as the desperate wonder weapons of world powers that were losing their grip, and their chance of victory. After the wars ended, a handful survived and remained in service, defending pockets of human civilization from hostile factions. The arcos stood, and they provided the tech and supplies to keep the mecha standing and the SPTs functional, which in turn kept the arcos standing.
Eventually the SPT started to fall out of favor as the assault mech came to the forefront. The SPT remained in it's role as a defensive lynchpin, but it then carried a minor stigma of being a primitive weapon system. The Atlantic Federation and Pacific Rim no longer construct SPTs, but they do maintain the units already in service. The Eurasian Alliance and ACPS still construct SPTs as their mech corps are still dealing with insufficient numbers to remove the vehicles from service.
Armor: Most SPTs have armor measured in feet of thickness. They are able to absorb staggering amounts of damage without serious or critical hits. Their sheer mass also reduces the effectiveness of many weapon systems as well. A round that will knock a mech to the ground might simply splatter against the armor or bounce off of it. An ABC sealed SPT could feasibly survive a near miss from a nuclear weapon, so long as it wasn't tipped over.
Firepower: Armed with not just powerful weapons, but often a large number of them, SPTs have an incredible ability to deal out damage. A standard SPT with conventional cannons is a rolling fire support base, while one armed with incandescent plasma cannons is a Zone Superiority Vehicle.
Weight: SPTs are heavy, and this limits where they can go. They are largely limited to sturdy terrain, and avoid mud, watery terrain, and such judiciously. An SPT can become mired rather easily. On the other hand, they are less affected by rough terrain, and can crush boulders, ruins of homes, and such with their treads easily.
Speed: SPTs are slow, they are among the slowest things on a battlefield that is largely dominated by speed and agility. Most SPTs offset this by being defensive in nature, and shooting at targets at long range with accurate weapons. Many SPTs assigned to an arco defense corps have a link up to the arco network, and will have long range sensors and targeting systems attached to the building, or will use spotter drones.
The SPT is a powerful weapon, often seen as an antique, or quaint compared to the flashy glamorous mecha piloted by the ace hotshot military celebrities. It still has a very important purpose, and that is a symbol of power and protection. In areas where the vehicles are no longer used, there will still be a few in VFW Display, decommissioned and open to the public to take tours of. In the militaristic patriotism of the Cosmic Era, such things are common. Military parks are common with arcoplexes, where the residents can get close to decommissioned hardware for taking pictures and celebrating military holidays.
In active places, the SPTs are symbols of safety and security. The vehicles are seen as extension of the city, and such are cherished by them. When a city's SPT goes on a patrol, people feel safe because that is their fighting machine, out there, looking for the enemy. When SPTs go on a patrol with mecha, its the heroes of the military fighting the good fight, by there is a symbol of the people there as well. The aces went in, but in spirit, the people have gone with them.
Take out the King, take out the Castle First: the SPT is a key point in a defensive strategy, and as such, the first step in breaking a line is finding a way to take out the SPT. The PCs have been tasked to shadowrun against an SPT, go in as undercover agents and take down the machine. Problem is the machine is a key part of civic festivities and sabotaging a vehicle in the middle of a celebration might be easier if there weren't so many people constantly looking at it, taking pictures, and such. Likewise the PCs might be tasked with running shadow security on the SPT to stop a group of hostile runners from damaging the vehicle. Its so hard to watch everyone near the tank when there are so many people in the park.
Objective Raid: in a mecha level engagement, the SPT is a slow moving heavily armored mech buster, and must either be protected from armor-busting infantry forces, or must be taken out by anti-armor assets. An SPT in an advantageous position can hold it's ground in an impressive fashion.