Starting January 28th, an armada of 11 battleships, 20 cruisers, 40 destroyers and nearly 1,000 carrier aircraft from 9 fleet carriers, began to rain hellfire upon guam. General Kuribayashi had realized a week prior from increasing airstrikes and more ships being spotted that the invasion was imminent, and ordered construction to cease. The men crawled into their start positions, and awaited the coming storm. As usual, the bombardment did little besides silencing some of the shore batteries, which still managed to sink the USS Burns.
The marines had originally been planned to land two points, south of the orote peninsula , and near Agana. However the concentration of Japanese forces in the south had meant only the later site was used. With only a few snipers and a handful of static positions. The marines were so perplexed by this that they took three days to advance towards the Japanese main line, suspecting an ambush. Some of the Japanese skirmishers in the north would outlast the entire battle, a few even fighting for years after the war ended.
Kuribayashi gave the order for the main fighting to commence on the 7th, and the marines fell under heavy artillery attack. The Aga-Yona line was nearly two miles deep, it's north consisting of machine gun nests and snipers, and heavier bunkers and pillboxes in the hills to the south. Kuribayashi had anchored his left flank with support from the east slope of mount Lam Lam, where the largest fortress on the island had been constructed. The entire mountainside had been practically hollowed out, dozens of artilery and machine gun positions overlooked the rest of the island.
The main fortres, nicknamed Kyoto Castle by the garrison, was inspired by fortifications on the Maginot line. Consisting of four different levels bulit near the summit, It held 3,000 troops and contained nearly five miles of tunnels. On level three were the centerpieces of the fort, two 28cm siege howitzers, nearly as old as Kuribayshi himsellf. The guns were still 30 feet underground, a small opening permitted 10 degrees of traverse and elevation, the range could be further adjusted by manually lowering the amount of propellant in the shells.
The fortress had steel doors and ventilation to protect against flamethrowers, and the terrain itself stopped tanks from getting too close. As it was also Kuribayshi's command post, it was equipped with radios and telegraphs to allow him to communicate with the island. The fortress had many access points, designed to also allow retreat when the time came, as well as an ammunition depot nearly 70 feet underground. The fortress had cost nearly 1,500 Korean and Chamorro lives to contruct, and would now become a bane to the marines.
The marine advance was constantly hampered as they clawed through the Yona line. Each post was supported at it's sides and rear, requiring a blooody assault, that often would find the positions already deserted. The bravest marines would sometimes try to use these tunnels to ambush the Japanese, only to find themselves pulled away and ripped to pieces by awaiting Japanese infantrymen. Many historians would later compare the ferocity of the fighting on Guam to the eastern front.
(Note: I edited the Casualty number's after some discussion that I couldn't really argue with)