The summary: you have to gain the most victory points, which you do by building houses in the castle or building buildings, with some minor victory points coming from others "using" your buildings more than you use theirs. Buildings are divided into several types, each type of which has to be built before the next type can be built, with some exceptions. Initial buildings produce goods or allow you to build the next type of building. Later ones produce money or victory points or more goods.
Each turn you collect income, place your meeples on buildings that you want to use that round (only one person can use any building each round), negotiate the position of a marker that will exclude some buildings from functioning this round, collect your rewards from the buildings, and build other buildings or build houses in the castle. Scoring the castle happens three times, after which the game ends.
Other games have just a few mechanics. Caylus seems to have taken just about all of them and rolled them into a sprawling collection. It's got every mechanic I can think of except trading: auction, negotiation, resource production, set collection, variable turn order, and so on.
While other games have a natural progression from "establish your source of income" to "collect your victory points", Caylus has paths that require three, four or five steps in order to do this. First you have to get these goods, and then buy this building, and then get these goods, and then buy this building, and then use that to get these goods, and then buy this building, and then you get a few victory points. And, no, I'm not exaggerating. And there are not only two paths, but multiple paths to do this.
While other games provide a means to establish a source of income at the beginning of the game, this game doesn't provide that until midgame or even later. The last few rounds then breeze by, while the vast portion of the game is getting to that stage.
For that reason, the game starts off much slower than other games and takes longer.
Now what do I think of all of this? I think it is great. But I can understand that it just goes right over the limit for people who don't think that "too many bits to keep track of" is a good mechanic. It annoys me as well. Ten different color meeples, and a multi-colored board, a road that meanders several times across the board so you can't easily tell coming from going. And the progression along the road goes: second part of the road, third part of the road, first part of road (castle), in that order.
Not to mention two different player order tracks, and four different favor progression tracks, all of which are half invisible during the game because your pieces sit right on top of the paths and on top of each other obscuring their benefits, rather than next to the paths so that you can still see what you will be getting this round. Bad game board design. It would have been far better to produce individual boards for some of these things, especially the favor track, instead of having stacks of marker disks falling over and hiding the tracks.
Well, those are the good and bad parts to the game. You can't tell from all of this, but I really enjoyed it. It was thoroughly complex and looked well balanced to start with. As usual, there are way to many things to do; not only can you not do everything, you can't even do one thing during a single round, as it takes a few rounds to collect enough to do anything at the start of the game. This was a hard concept to wrap around for some of our slower players. And exhibited some rude hastening comments or eyeball-rolling from some of our quicker players.
In our game, most of the second, third and fourth sets of buildings were ignored, and we all did most of our victory points in the castle. Aside from the very first round, we also did almost no marker positioning in order to block buildings from functioning. Maybe it is our group and we are not nasty, or maybe it is just not worth it in a five player game to expend energy hurting one other person rather than moving ahead, or maybe none of us every earned enough income to make this feasable. This step in the round may as well have been dropped.
There was also one "building" in the game that didn't appear to have any use, the Gate, which allows you to store a meeple during the "auction" phase and then move it at the end of the auction to any remaining unoccupied building. The theory is that it allows you to hide your plan and also place it where you end up needing it most. But in practice, all of the good buildings were occupied at the end of the auction anyway. It might have been more useful if it happened after the marker negotiation, so that it ensures that your meeple doesn't go to waste, but that's not the way it works.
Elijah had to leave in the middle, and it was possible to continue playing without him, which is a good feature. We still had to end the game without finishing it. I was somewhat ahead as was Itamar, but it was all around too hard to tell what would happen by the end of the game.