This was a great episode! It wasn't unbelievably amazing, but it didn't need to be. I personally quite liked it, especially the humour, and I thought the themes worked well in this episode. While the moral was a little predictable, it being so didn't dampen my enjoyment of the episode overall.
In "Castle Sweet Castle", Twilight's friends try to make her new castle feel more like home after finding out she's been avoiding it all day, which is a solid idea for an episode, and it ties together the last season nicely.
The solution to the problem is where the fun begins! In the first episode of the season, we are told that the mane six have spent a year together now, so you'd expect Twilight's best friends to have some idea of how to go about making the castle homely. While they do come out triumphant in the end, the journey there is quite amusing. The five of them spend the entire episode trying to put the castle together with personalised decorations, only to realise that they've made it feel too much like their homes rather than Twilight's, resulting in a cluttered mess. They go through the tedious process of un-decorating the castle, followed by a trip to the Golden Oak Library, all while Spike distracts Twilight until sunset.
During the decorating, we get a song, "Make This Castle a Home", which compliments the episode nicely.
In the end, they come up with quite a brilliant addition to the castle which wraps up the "Twilight's home was destroyed" storyline perfectly! Although, I wonder about where the citizens of Ponyville are to aquire books now.
I've read comments from others saying that the plot and themes of the episode would've worked better in season one, or that the conflict should have been resolved much quicker since the characters have known Twilight for so long, but I tend to disagree with those statements.
While each of the characters has gotten to know Twilight over the past year, they've also had that same year to get to know four other ponies. That's a lot of new close relationships! From personal experience, I would argue that getting to really know even one person actually takes quite a lot longer than that, let alone several. It is because of that thought that I find the characters' struggle to make the castle homely very believable. A year sounds much longer than it actually feels.
Overall, this episode did its job and it did it well. The jokes were funny, the song was nice, the callbacks to previous episodes in season one were cool, I liked the moral, and the ending to the episode, as well as the episode as a whole, was solid.
Season five is going strong, and I hope it keeps going as well as it has for these first few episodes!