Single player, by 
Such an immense level would seem to warrant a similarly immense review… it took me 80 minutes to complete. But the level isn't all that variable, so it's possible to summarize. You're wandering through an immense tower, trying to reach the top, but before you get there you want to collect 600 coins, and that's going to take forever, so really most of the time you DON'T want to reach the top. Rules like "always turn left" or "always turn right" do help (moreso the latter), but there are occasional rooms surrounded by loops which you have to visit intentionally, and occasional one-way passages due to gravity, so navigation isn't always automatic, you do have to pay attention sometimes. There's generally a visible distinction between passage and room, and just about every room has one or two coins, but rarely more than that, so there's a lot of level here.
Along the way, you pick up just about every ammo type, usually isolated to specific areas of the tower. There are occasional fastfire pickups, but those are useless given how many enemies you need to kill before you can get there. There are also a good number of shields, invincibility carrots, birdcages, and apples to keep things varied and make sure you feel rewarded as you wander the endless tower.
Really, the best decision being made here is probably how easy it is, as a result of how the game works: if you ever died, even if you had 599 coins, you'd lose 100% of your progress. No player is going to want to start from the beginning. So the level has obstacles, absolutely, lots of spikes and enemies, but they're all very visible and there are plenty of carrots. Again, you have to pay some attention, but you're not being given insurmountable challenges. (The latter levels even demonstrate the author CAN do harder stuff than this, but chose not to.)
The music's fine in my opinion, but since you're going to be here for an hour or more, there's basically no track that will keep you occupied that long on loop. Turn it off and listen to a playlist.
Ultimately The Queen's Tower is a form of meditation. You're doing the same things against familiar enemies in square passages, on and on, with occasional breaks in scenery. There are distinct areas with distinct gimmicks, such as spike balls or glowing lava pits, but mostly you know the drill. Sit back, hold down the fire key, and relax. I think the level might actually work better if it were linear so you didn't have to worry about making choices about which direction to go, one less thing to think about, but I was successful, after all, so probably you can be too.
The second level is a boss fight, it's not too exciting. The third level I like a lot, even absent the context of the broader pack. It's just really good stuff that doesn't overstay its welcome but still poses fun platforming challenges with a cool unique aesthetic. Very good. The fourth is puzzles. Basically every level in this is totally different from the rest. I mostly just talked about the first because it's the main attraction and super long, but I do like the third quite a bit.
I can't fathom how long it must have taken to make this, by the way. Some rooms are more creative than others, certainly, but every room seems to be different, not copied-and-pasted at all. The graphics aren't especially advanced, just blocks in brick patterns, but again, the level is ridiculously huge… doing anything more than a repeating 1×1 pattern is impressive.
