Doodle Blocks is genuinely two separate games sharing one page. Run mode is an endless isometric parkour sprint across a hand-drawn block world -- you're always moving forward, and the only decisions are which lane to be in and when to jump. Build mode drops the timer entirely and hands you a blank block grid to paint however you like, with zero risk of falling.
What makes Run mode work as a skill challenge is its simplicity: gaps appear one or two lanes wide as difficulty increases, and landing in one without having jumped ends your run instantly. There's no combat, no upgrades, no currency -- just lane-switching and jump timing against an endless, gradually harder course, with your best distance saved locally so you always have a number to beat.
Because gaps can appear back-to-back, it pays to commit to a lane early rather than weaving right before a jump -- panicked last-second lane switches are the most common way to land in a gap you could have cleared. In Build mode there's no scoring at all, so treat it as a sketch pad rather than a challenge -- it's there to unwind with after a hard Run attempt.
Run mode is the actual game -- an endless, ever-scrolling parkour course where you dodge gaps for distance and can fail. Build mode is a separate, fail-free creative tool where you paint your own block layout on a grid, with no timer, scoring, or way to lose.
The run ends immediately. There's no health bar in Run mode -- one missed jump into a gap is game over, and your distance for that run gets compared against your saved best.
Build mode is meant for casual, in-session creating rather than long-term projects -- your saved progress in Doodle Blocks is specifically your best Run distance, tracked locally in your browser.