It used to belong to a lich who, in his mortal days, was an ally of Strahd and assisted him in his initial conquest of the valley.
The tower is enchanted with an anti-magic field and is warded against unauthorized entry by a set of glyphs etched into the front door. The door is unlocked by a series of gesticulations which members of the party have memorized.
There is an elevator of sorts that travels between the 4 floors of the tower. It is controlled by a series of chains and pulleys that are operated by magical clay statues and activated with command words.
The first, second, and third floors of the tower are largely empty and unadorned. The fourth contains Van Richten's residence and cache of items, including a large chest, a suit of armor, and several scrolls.
Van Richten has abandoned the tower as a residence, fearing that Strahd has learned of it's whereabouts after the party fled there following an encounter with the vampire.
A wagon belonging to Esmerelda, and loaded with a great deal of explosives, was once parked outside of the tower. But the party was able to convince Mordenkainen to take the wagon into his pocket dimension and thus secure for the party a large, mobile bomb.