Metal Fortress (メタルフォートレス Metaru Fōtoresu) are one of the four tribes from the Blue World.
Z/X Encyclopedia Entry
The latest technology, introduced without regret, is a humanoid battle weapon that is driven by multiple people. They are the so-called "Giant Robots". With fighting strength that matches how it looks, the Metal Fortress are said to have influenced the way battles were fought in the Blue World. But the most terrifying thing is the camouflage technologies to deceive enemy forces.
They can turn into buildings.
They're stored underneath parks or ruins
They're a fusion of several smaller machines such as vehicles that combine with each other.
While the above listed are just a few examples, the total number of Metal Fortress in the Blue World is unknown. And with all those disguises, it's not hard to imagine them being sent to Present World.