You're right that it's not verb serialization, but 去 is still a verb in this sentence. Instead, 怎么走 is the interrogative adverbial phrase that modifies the V+O 去故宫. Consider the sentence structure of a possible answer to the question:
我 骑车 去 故宫。
S Adv. V O
The structure needs no explanation. Now replacing the adverbial phrase with an interrogative one, the sentence becomes an interrogative sentence:
我怎么走去故宫?
Invert the sentence for emphasis:
去 故宫 我 怎么走?
V O |S Adv.
In the case of 故宫我怎么去, it's the inversion of 我怎么去故宫, where the interrogative adverbial phrase is simply 怎么 without a verb.
我 怎么 去 故宫?
S Adv. V O
故宫 我 怎么 去?
O |S Adv. V