TCO18 Marathon Round 1 RoadsAndJunctions started on May 9th and will run for a week. You can find it in the list of active contests. I am the writer :-)
This year the online part of the Marathon competition consists of 4 main rounds, each running for a week. The winner of each main round advances to the finals, and so do 8 participants who accumulated the most points across all main and lightning rounds. 150 competitors who accumulate the most points get t-shirts; given that each round awards points to top 50 places, it's quite possible that the t-shirt condition will transform into "top 50 in each round get t-shirts".
The round has begun! :) You can register and compete now: https://community.topcoder.com/longcontest/?module=ViewProblemStatement&rd=17153&compid=63555
Check out the current standings here: https://community.topcoder.com/longcontest/?module=ViewStandings&rd=17153
Are there any prizes?
As the blog post says, TCO18 T-shirts for a certain number of top finishers (up to top 50, but the exact number will be known after all online rounds).
If I understand correctly, we need to participate in all four rounds to accumulate points. Is that true?
No, points for each round are awarded independently, you can participate in as many rounds as you want to.
"150 competitors who accumulate the most points get t-shirts" ?
This doesn't contradict the possibility to skip a round. If you don't participate in a round or don't place in top 50 in it, you get 0 points, not NULL, so the points accumulated in other rounds are not turned into NULL upon addition :-)
It makes sense to participate in all rounds to get the best chance at accumulating enough points, but it's not a hard requirement.
An editorial is available.