Castle likes to do theme episodes with Castle as the intermediary between the police and a specific group of people. "Punked" focuses on steampunk genre, which seems to be growing these days. From what I could tell, quite a bit of money was shelled out for authentic looking sets and costumes, adding to mood that these people are heavily involved.
The episode has a predictable twist, given that there was plenty of time left in the episode and the criminal has to be caught at the very end, but it didn't matter too much. Esposito's condition was funny and Alexis is always cute with Castle. And Victor Webster shows up to the police station to take Kate out. Cue ominous music!
In the end, there's a very nice tribute (using the logo) to Stephen J. Cannell, a writer in the purest sense (unlike a lot of producers these days).