Note to readers: This is a session re-cap of an ongoing D&D 5th edition campaign set in a loosely adapted version of N. Robin Crossby's Hârn. A journal of sessions 1 through 54, beginning in February 2015, can be found here.
In-world dates: 5 Ilvin 720 TR [first week of winter]
Moon: waxing crescent (full moon in 9 days)
Opening the door, Juan and Dracul discover within a wizard's study and laboratory: the walls are lined with shelves full of arcane jarred or dried ingredients (eye of newt, toe of frog, etc.), some utterly unidentifiable. In the middle is a large workbench with alchemical apparatus and a variety of instruments. On one end of the table sits a small, ornate birdcage containing a mouse. Beside it, lying open, is a large book. On the visible page is an illustration depicting a birdcage that closely resembles the one nearby. In one corner of the room, there is a 10-foot-diameter raised dais of ice with low steps leading up to it; in another, an extremely lifelike ice statue of a female human warrior. The latter looks strangely familiar...
While Juan ransacks the room for things of value (after taking care to wedge the door closed), Dracul examines the statue and realizes it's Alexandria, the Amazon warrior you had encountered previously, both in waking life and in the odd collective dream you had of a labyrinthine ice tower. Activating his Searing Sword, the dragonborn warrior carefully thaws out the frozen Amazon. Meanwhile, Juan hears movement outside and takes measures to secure the door. Peeking through the keyhole, he sees a slim figure robed in white approach the door, try the handle and, finding the door barred, bend down to peer through the keyhole. He thus comes eye to eye with a pale woman whose emotionless expression is as cold as the aura she exudes. She straightens abruptly and leaves.
Released from her icy prison, Alexandria explains that she was captured while investigating the tower and imprisoned in ice by the ice queen herself several days ago. Though unable to move or speak, the Amazon was nonetheless able to perceive, dimly, some of the activity around her. The ice dais, she notes, seems to be a teleportation device of some kind through which the ice queen occasionally enters and exits the laboratory. She also observed the ice queen repeatedly speaking to the mouse in the birdcage, but the latter never replied.
Dracul sets about performing the ritual magic that will allow him to Speak to Animals while Alexandria and Juan make preparations against attack, either via the door or the teleportation dais. As a precaution, Juan unfolds the Well of Many Worlds and spreads the silken fabric atop the dais. Sure enough, moments later, the ice queen materializes on the dais... and promptly plummets through the Well!
Meanwhile, Dracul engages the mouse in dialogue. It is (as your dream suggested) none other than Baba Yaga herself. She does not know how to free herself from the birdcage; presumably Elvanna, her daughter, knows a command word. The cage itself appears to be indestructible. Baba Yaga inquires after her hut, but you do not know its whereabouts.
There is hammering at the door: guards and wolves! Time to go. Leaping onto the teleportation dais, you speak the command word — Alexandria and Baba Yaga have heard the ice queen using it – and are instantly conveyed to a sumptuously decorated bedroom: a vast ornate bed draped in the finest white silks, fine tapestries, and a large locked chest occupy the space. In the corner stands another ice statue, this one of a young girl in peasant garb. While Dracul sets about thawing the girl, Juan picks the lock on the chest, inadvertently setting off an icy explosion! He is rewarded, however, by discovery of several scrolls and a superbly crafted arrow within. Alexandria searches for another exit, and finds one in the form of a secret door. Unable to figure out how to open it, Juan resorts to hacking at it with his crowbar. The girl, it turns out, is named Heidi, and asks to be returned to her mother in the village. Baba Yaga, however, tells you Heidi is very likely Elvanna's daughter (and thus her own grand-daughter).
Two hundred feet below, at the base of the ice tower, Gil'Doren lands softly (thanks to Feather Fall) and finds himself confronted by two very confused guards at the tower entrance. Snaring them in a magical Web, the tall elf races inside and begins a hasty search for a route leading to the upper levels whence he fell. Bypassing several doors, he notices a secret door which he succeeds in opening: it leads to a securely locked vault and to some kind of administrator's office. Realizing that he is being hunted by the guards, he Dimension Doors to the tower's parapets and, seeing no one, begins to descend the stairs. He passes a section of wall through which come sounds of chopping and hammering, notices a secret door, opens it and.... finds himself face to face with Juan, Dracul, and Alexandria.
Reunited at last (but without Cade, who was last seen being processed by a mechanical monster), you return via stairs to the lower levels and duck into the passage leading to the vault. Juan succeeds in picking its three locks, but is jabbed by a poisoned needle. Inside, you discover a great deal of wealth – all mundane items, but of considerable value. Safely ensconced within Leomund's Tiny Hut, you settle down to rest and plan your next move.