Bigger Zip grids reward thinking before drawing. These techniques let you work out large parts of the path without guessing.
1. Forced corners and edges
Any cell with exactly two open neighbours (corners, and cells boxed in by walls or by your path) must use both of them, unless it's the start or end. Mark these pairs mentally first. Chains of forced cells often run along a whole edge.
2. Dead-end detection
Only two cells in the whole grid may be ends of the path: the 1 and the last number. Any other cell that drops to one open neighbour means your current path is wrong. Back up until it has two again.
3. Don't split the grid
The unvisited cells must always stay in one connected piece that also contains the next numbers. If a move would split them, it's a mistake, even if every piece looks fillable.
4. Count the steps between numbers
If the path goes from 6 to 7, the number of cells it visits in between can't be fewer than the shortest distance between them. When few cells remain, the gap between numbers tells you whether the path must go straight or wind back and forth.
5. Checkerboard parity
Colour the grid like a chessboard. Every step changes colour. So between two cells, an even number of steps always lands on the same colour, and an odd number on the opposite colour. On an even-sized grid, a path that fills every cell must start and end on opposite colours. When the counts don't work out, a route is impossible.
6. Walls change everything
A wall turns open space into a corridor. Cells beside walls often become forced (technique 1). Expert grids use walls to replace many numbers, so look at them first.