Caylus is a bit complicated to describe, as there are several competing elements to the game which interact in ways that seem, on first inspection, a touch fiddly. However, imagine, if you will, that Richard Breese's Keydom (remade as the more well-known Aladdin's Dragons) was split into two parts: the bluffing part of secretly allocating your strength to various actions, with the player who allocates the most getting to take the action; and the semi-strategic part of deciding which of a variety of actions to go for. If you take the bluffing part, you get the foundations for Ys, which had the secret power allocation for collecting resources but not much in the way of special actions. If you take the special actions part, you get the foundations for Caylus, which has no bluffing.
Each turn there are buildings available on the board for you to allocate your workers to, each of which has some special power – acquiring more resources, building more buildings, making money, trading money for resources or vice versa, acquiring victory points, altering the tactical play of the game, etc. Each turn, players iterate on placing workers onto the board, each of whom can operate one building (and each building can be used only once per turn). The goal is to acquire resources to build bricks in the by-now standard Castle or Cathedral (for the record, it’s a Castle in this game). This is just the core, and there is a fair amount of additional chrome to the game, but it's primarily just a bit awkward to explain and the game actually plays pretty cleanly once you get going.
I liked Caylus. It seems to me not an obvious long-term classic, but it's got a lot of good stuff going on, and there are tough and interesting choices. It's still got a fairly weak theme, but it's more flavorful than Ys was and less prone to analysis paralysis. I like how they've supplied many more different buildings than you could possibly use in one game, so it's clear that games will develop rather differently based on the mix people build. I like how you have to plan a couple turns in advance to get anything done. I like how early choices really matter and drive the later-game.
I'm less enamored of the playing time, which was a bit too long. The box says 60-150 minutes, and I'm pretty sure Caylus wants to be on the very low end of that, and certainly should not be pushing the high end. If the game routinely take 2+ hours, I think Caylus will be on the flea market table sometime next year despite its charms. But if the playtime can come down to 60-90 minutes (possible), this might be one with some long-term replay value. My sense is that like most small-press games, this one isn't quite where it needs to be, and probably needed a touch more streamlining (and some traction on a plausible theme would have been a bonus); but what's there definitely works, and is interesting. So while I can say with confidence that this is not the next Puerto Rico, it's worth checking out. I'll let you know as I get to play more myself.