This is one of those tropes where the protagonist is walking a razor's edge every single day. She's technically protected by a contract, but that piece of paper is only as strong as the emperor's current mood and the political winds. It's less about her having power and more about becoming an expert in reading where the power flows. She has to identify who wants to use her as a pawn, who genuinely might become an ally, and who is an active threat that sees her contract as an insult to their own faction.
In a lot of the stories I've read, the successful contract concubine isn't the one who fights head-on. She's the one who turns her supposed weakness into a weapon. Her low status means nobles might underestimate her, letting her overhear crucial plots. Her contract might stipulate she gets no royal heirs, which ironically makes her a safe confidante for a crown prince who doesn't want another rival mother scheming behind his back. The real struggle isn't a physical battle; it's a continuous, subtle campaign of information gathering, strategic favors, and carefully measured displays of loyalty—or disloyalty—to keep multiple powerful parties balanced against each other. I find the tension comes from knowing the contract could be voided at any moment if her patron dies or falls from grace, leaving her utterly exposed.
Honestly, the ones that hook me are when the concubine starts building her own web of influence from the bottom up, using maids, eunuchs, and overlooked minor officials, creating a power base the court never sees until it's too late.