War Elephants and Incendiary Pigs
The ancients didn't have access to tanks, amphibious or otherwise, but what they did have was elephants. And of course, human nature is such that as soon as people see a giant, lumbering beast, the only thing they're wondering is the best way to use it to murder other people.
"Oh yeah, that might work."
Elephants were used in warfare throughout the ancient world, most notably by the Persians, who captured them from India. The rationale was simple -- elephants are huge and intimidating, with tough hides that arrows just bounce off of, and spears attached to their faces. If you're an exotic foreign army, the best way to frighten the Roman legions is to remind them that giant monsters are on your side, and unfortunately for the enemy with their pansy-ass swords, you can't just bring a knife to an elephant fight.
But war elephants weren't just a relic of ancient times -- armies used them right up to the end of the 19th century, in part due to their ability to withstand gunfire, and as shown in this 1893 photograph of the Siamese army, they could be equipped with cannons. Let's see a horse do that.
If we'd spent all that F-35 money on war elephants, we can guarantee you it wouldn't have been a waste.
Unfortunately, elephants come with a rather serious design flaw -- despite their size, they're huge pussies, and when you spook them, they don't really care whether they're squishing the humans on one side of the conflict or the other. So although they might be able to shrug off the enemy's weapons, there's a better than good chance that your elephant is going to go into a fear-induced stomping rage among your own ranks if it gets shot enough times.
So yeah, maybe not the best place to build a tower.
The Greeks came up with a particularly novel (though horrifying to animal rights activists) way of exploiting this fact. According to them, the elephant's natural enemy was the pig, although cartoons always taught us that it was the mouse. When they were besieged by Macedonian armies with their war elephants, the Greeks took the obvious course and coated a bunch of pigs with tar, set them alight and set them loose among the invading forces. Then they just sat back with a box of cigars and watched the Macedonians getting stomped on by their own terrified elephants.
Historians are divided, but there's some evidence that ancient armies might have used rhinoceroses to combat elephants as well. One report from Portugal in 1513 describes rhinos as "the mortal enemy of the elephant," being sent in to attack elephants with their face horns. Kind of like in 300, although we're sorry to say that most academics don't regard that as a reliable historical source.
For more crackpot weapons, check out 7 WTF Military Weapons You Won't Believe They Actually Built and The 7 Most Stupidly Overpowered Hunting Weapons.