Lassar was a Zarosian fortress built on what is now known as Ice Mountain. Nothing remains of the fortress; it is assumed to have been destroyed during the God Wars. It may be that the black knights rebuilt the ruins into their own fortress, or built their fortress by cannibalising materials from the ruins, explaining why there are no other traces of ruins on the mountain.
Lassar, during its years of service in the Second and early Third Ages, was a massive fortress on the edge of Zaros's main kingdom. Its main features, according to Azzanadra, were impressive spires. It existed atop what is now known as Ice Mountain, a large mountain in northern Asgarnia. At the time, Asgarnia is believed to have been much colder than it is today, yielding snow year-round; yet nearly all of it had been settled by the Zarosians.
Lassar was likely near the edge of Zaros's largest kingdom, which, at the very least, encompassed the entirety of the Wilderness, Misthalin, and Asgarnia (although it is speculated that it could have included nearly all of Gielinor). Lassar most likely served primarily as an outpost, which could protect the area against invaders. It is unlikely that it served as a realm of transport, as Ice Mountain was far more dangerous than it is today.
When Zaros was overthrown by the Mahjarrat Zamorak, his kingdom was beset by many religious and racial factions that had been at war with his empire for many years. Lassar was most likely destroyed within the first thousand years of the war, although by whom is unknown.
Today, no ruins remain of Lassar, and the fortress itself has been forgotten by all but a few. The only remains of Lassar may be the icefiends that wander the area.
In the Fifth Age, it was discovered that those with knowledge of the Ancient Magicks could teleport to the area where Lassar once stood, using the Lassar Teleport spell, with 72 Magic, two law runes, and four water runes.