Your goal is to guide all the Chus to a portal by placing arrows on the playing field. Complicating this endeavor is the fact that there are two kinds of Chus — red and blue — that can never be allowed to meet. And if you think you can rely on the walls to guide both types along the same path, think again! Red and blue Chus turn opposite ways when they hit a wall. Even worse, there are evil Chu counterparts, the green and yellow Nyans, which must be kept clear of all Chus as well as the portals.
tChu is most definitely a planner's delight. Once you place the arrows and start the Chus, that's it — if you want to make changes you'll have to start the level over. You'll probably have to do that a lot anyway. The early levels aren't difficult, but it takes a while to get used to the Chu behavior, and the later levels will have you resetting constantly as you encounter many unforeseen collisions. Usually, levels are designed to take advantage of the different handedness of the two Chus, but the more devious ones turn it against you and make you overcome the natural Chu tendencies.
Unfortunately, there is a bit of an oversight which interferes in the planning of your solution. If you do allow red and blue Chus to meet, or to be eaten by a Nyan, or if a Nyan reaches a portal, the level resets immediately. Sometimes this forces you to deal with the smaller problem of avoiding collisions first, rather than focusing on the larger problem of planning your path. Maybe it's just the way my mind works, but I'd prefer the Nyans and Chus to keep running even if I've already failed a level, in order to see they will behave later on. Still, it's not a deal-breaking problem, and it doesn't overshadow the many difficult but great levels just waiting to be solved. With 90 of them plus a level editor, it's enough to keep a person busy for quite a long time.