Overall from 10pm-11pm, Castle got a 3.3, which is above my line of "complete failure sure to be cancelled" but that was under the assumption of a low 5 rating, not a high 5 rating. This is still very borderline as a result for the premiere. Finals will drop because DWTS ran until 10:02pm. But regardless, the 10:30pm dropoff is concerning (22% left, 3.7 to 2.9) and retention of 56% overall is never a good thing. We'll see which direction the ratings/retention go next week (I'm 99.9% positive the DWTS lead-in will be lower...) and my guess for Castle based on these half-hours is down, down, down.
Unfortunately, the return of DWTS laid the smackdown on the other networks, with shows that were showing some great momentum losing it (Big Bang Theory at a 3.6, How I Met Your Mother at a 3.5).