Fortress Maximus was an architect in his early career, designing and even constructing many works on Cybertron. When the war came, he reluctantly turned his talents to fighting.
During the long absence of Optimus Prime, Megatron, and their respective troops from Cybertron, unit commander Fortress Maximus became so sick of the war that he decided to leave it. He didn't care if it made him a coward or not, he simply believed that the war would go on regardless of whether or not he was in it. Taking a number of Autobots with him, he retreated to the peaceful planet of Nebulos.
Unfortunately, it wasn't long before mistakes were made and Maximus's group found themselves at odds with the local human populace. Maximus 'proved' their peaceful intentions by marching up to the Nebulan capital, and discarding the group's weapons. When that didn't work, he did the next logical thing and pulled his head off. Four other Autobots also resorted to this grotesque stunt before Galen, head of the Nebulan World Watchers, was convinced they weren't a threat.
When the Decepticons followed the Autobots to Nebulos, Galen himself ended up becoming the head of Fortress Maximus to help defend the peace and his world, as both the Nebulan and Autobot underwent the binary-bonding process to become a Headmaster. Fortress Maximus remained the leader of the Nebulan Autobots for the rest of their battles there, eventually abandoning the planet to support his stranded comrades on Earth. During the transport, Fortress Maximus underwent a number of treatments to alter his size considerably, increasing himself from a city-sized behemoth to a slightly larger city-sized behemoth. With these changes, he could save a lot of time in combat by simply stepping on Decepticons instead of having to actually fight them.
Of course, with his larger scale, Fortress Maximus needed a new head (Galen still being stuck at roughly human-size). For this reason, another Autobot, Cerebros, was added to serve as a secondary stage between Galen and Maximus. As the head of Cerebros, Galen could transform Cerebros into the larger head of Fortress Maximus.
Arriving on Earth, Fortress Maximus led his fellow Headmasters to Mount St. Hilary, source of the Autobot distress call that had drawn them to Earth. After recovering the damaged Goldbug, the group encountered Spike, who had come in search of his missing younger brother Buster Witwicky. Spike asked for the Autobots' help in finding his brother, but Fortress Maximus was more interested in their primary mission of locating the Autobots of Earth, and led his troops off to follow the fuel trail of the missing Ark.
Scorponok's crew soon followed Maximus to Earth, however, and to the same cave. When Spike replayed Goldbug's distress message, Fortress Maximus and company returned to the cave, apologizing to Spike for his failure to consider the danger he had brought to Earth's natives by leading the Decepticons there.
In the ensuing battle, Spike retreated further into the volcano's caves to seek shelter. Scorponok followed, seeking a hostage; Fortress Maximus pursued him. As the caves narrowed down, Maximus was forced to send Cerebros onward, and eventually Galen himself had to leave Cerebros to cross a fragile rock bridge. Scorponok used the distraction to blast rocks down on Galen, killing him. As the Decepticon leader left, laughing, Galen passed on his control helmet to Spike, allowing the Earther to assume control of Fortress Maximus and drive the Decepticons off.
Spike soon underwent the full binary bonding process and became the leader of the Steelhaven's crew of Autobots. He tried using this power to rescue Buster from Club Con, but ended up damaged, though he did manage to send Shockwave falling to his apparent demise. They then caught wind of a Floppy disk in possession of one Ethan Zachary, which contained the memories and personality of Optimus Prime. With the aid of a computer generated version of Prime, Fortress Maximus gathered a lot of data from Scorponok's Pretender experiments, and gave that ability to some of his own crew.
Unfortunately, Grimlock developed an immediate dislike to Fortress Maximus, partially because he was a possible threat to Grimlock's leadership, partially because he was part human, and partially because he had a fugitive (Goldbug). He challenged "Fruitloop Multipuck" to a duel for leadership on the moon, but his injuries from prior battles forced Fortress Maximus to have the recently liberated Blaster fill in for him. When Ratbat led an assault on the Autobots during the duel, Fortress Maximus ordered Goldbug, Joyride, Getaway, and Slapdash to take the disk to Nebulos to restore Optimus Prime.
Spike and Fortress Maximus would go on to have a very tumultuous relationship, fighting both the Decepticons and, at times, each other, as Spike constantly expressed his displeasure at being the large Autobot's partner. Once his brother Buster was finally rescued, Spike's organic qualities was instrumental in preserving Fortress Maximus from an Underbase-powered Starscream in the battle of Buenos Aires. Spike tried to go his own way and resume his life, but time and time again the Decepticons would pose some sort of threat to the planet so immense that he would have to join with Fortress Maximus to combat it.
After the other Autobots and Decepticons left Earth to battle Unicron, Spike and Fortress Maximus remained on Earth to safeguard the planet. Thus, when the remains of the Ark fell from orbit and crashed in the northern tundra of Canada, and Galvatron emerged from the wreckage to wreak havoc, it fell to Fortress Maximus to stop him. After putting aside their differences, Spike and Fortress Maximus managed to unite in thought and deed, and defeat the crazed Decepticon.
It was ultimately Megatron who would prove to be the undoing of this giant warrior: when the reborn and reconditioned Decepticon leader stole the Ark and used it as his personal flagship, Spike and Maximus took it upon themselves to stop him. Brutally beaten in combat with an upgraded and super-charged Megatron, the pair dragged themselves to the heart of the ship and threw themselves into the anti-matter containment unit at the core of its power supply, killing both of them. To add further insult to their sacrifice, it was a complete waste of effort: every other Transformer who was on board, including Megatron, managed to survive the destruction of the ship and lived to fight another day.