In spring of 1552. begun a civil war between Oda Nobunaga and Oda of Kiyosu in Owari, so in 1554, the Imagawa clan came to the west and built the Muraki Castle in the southeast of Owari, besieging one of the Nobunaga's vassals, Mizuno Nobumoto (uncle of Tokugawa Ieyasu), in his castle of Ogawa, while another one was persuaded to surrender the castle of Terumoto, so that Ogawa was cut off from the rest of the Nobunaga's territory.
The, date of the battle, however, is said to have happened in late January. Did they build an entire castle in just a month?
I seem to remember a (probably mythological) story of Toyotomi Hideyoshi building a castle in a single night. While that probably didn't happen, he still built that castle pretty fast didn't he? How quickly could one build a castle at this time? And was it a big investment for the Imagawa or were they closer to forts?
The story of Hideyoshi's castle comes from the Battle of Odawara, fought in 1590. Hideyoshi, who at that point controlled most of Japan, turned up in Odawara with a massive army, and laid siege to the city. His troops - tens of thousands of them - got to work building a castle on heights overlooking the city. It didn't take them one night to build: it took them roughly 80 days. But the castle was built in the middle of a forest, and when it was finished, the forest surrounding it was cut down in a single night, so that to the defenders in Odawara below, it appeared as though it had suddenly appeared in a single night.
It's quite an impressive castle, given how quickly it was built, and the ruins of it are still there. It was the very first castle in the whole of the Kanto region to be built with stone walls, as the techniques for that were still comparatively new, and had yet to spread to eastern Japan.
Another notable construction of Hideyoshi's, conducted in the middle of a battle, was the dyke built during the siege of Bitchu-Takamatsu castle. It wasn't a castle, but rather a wall, several meters high, and about 4 kilometers long, which effectively dammed a river, and caused it to flood the castle that Hideyoshi was attacking. The dyke was built in a few weeks, while under fire from the enemy inside the castle.
Sadly, most of the dyke was destroyed to make way for a railway, but a small section of it still exists, having survived from when it was built by Hideyoshi's troops in 1582, in the middle of a battle.