3/3/3
If tower < 15
Stock: +15
If tower > 50
Wall = 150
Always:
Tower = 15
The stock and tower gain is very cheap, especially if used with a high wall. This after a play or two of Road to Valhalla would give incredible gains. Whether this is too much for a Rare, though, I can't say. It is conditional, after all. (By comparison, Prosperity gives 16 stock for zero cost, and its only condition is that it has no effect when New. Granted, it has no alternate effects as this one does.)
The effect if tower is above 50 (increasing wall at the cost of 36 or more tower) doesn't seem out of line. You're trading one potential weakness for another.
As a card that primarily performs building effects, I wonder if it shouldn't have a cost that draws mostly from Bricks rather than a flat cost. That way, the stock effect would be more of a resource distribution from bricks to the other two, ideally to fund a large attack.