Since there are a bunch of new players right now, I thought it might be useful to share some tips and strategies.

My first tip would be to try and not to make any assumptions. Only place a line or a cross if you know for certain that the section must be one or the other. If you're approaching the game like a jigsaw puzzle where you're moving pieces around to see where they'll fit you're going to struggle once the puzzles get bigger.

Here's some useful strategies starting with the most basic to more advanced:

1 - // Dealing with 0s //

A cell with a 0 indicates that there are no lines on any side of the cell. This means we should place a cross on all four sides.

Since we are working on creating an endless loop, if a 0 is on the side or corner of the board there are also sections that would lead to a dead end. These can also be marked with a cross.

2 - // Avoiding splitting //

If placing a line will cause a split in the loop then we can place a cross instead.

3 - // Removing dead ends //

A formation of crosses like this mean that there can't be a line leading in to them, so we can place another cross instead.

4 - // Adjacent 0 and 3 //

When a 3 is next to a 0, we know which one of its four sides is not a line. So we can draw a line on the remaining three sides.

All lines must continue and there is only one direction these lines can continue. Let's add those.

The loop never splits and there are four sections that would cause a split so we can add crosses to those.

5 - // Avoiding multiple loops //

If placing a line will create a loop when there are other lines or uncompleted cells elsewhere on the board, we know that there cannot be a line there. We should draw a cross instead.

6 - // Constraints on a 1 //

When a 1 is backed in to a corner as shown on the left, placing a line in that corner would mean that it would be unable to continue. We should place crosses there instead.

7 - // Constraints on a 3 //

These 3s could be said to be in a corner with the two crosses diagonally.

There are four different formations for three lines on a cell, and only two of them would be possible in this scenario (shown on the left). No matter which of these two are the correct, two of the lines are always correct so we can draw those in.

8 - // Adjacent 3s //

When there is a line of 3s adjacent to each other the line can only "snake" between them in one of two ways (shown on the left).

No matter which of these two variants end up being the correct path, they will always be separated by a line with lines on either end.

This happens no matter how long the line of 3s is.

On top of this, there will always be crosses placed in the shown sections to prevent the loop splitting.

9 - // Any number in a corner //

No matter what number is in the corners of the puzzle we can always use it to draw new lines or crosses.

A 0 can't have any lines touching, and this will create sections that would cause a dead end. This means we can draw crosses there instead.

When there is a 1, drawing a line on the outside of the puzzle would mean that for it to continue it would be touching the 1 cell twice. So we can place crosses here.

With a 2 there are six different possible formations. Only two of these formations are possible when the 2 is in a corner. Both options will result in the loop continuing on the path shown above.

With a 3 there are four possible formations. Only two of them are possible in a corner. They both will have lines drawn on the outside of the puzzle.

10 - // A line reaching a 3 //

When a line is pointing towards a 3 there are only two formations of lines that the cell containing the 3 can be (shown on the left).

No matter which of these two are the correct, two of the lines are always correct so we can draw those in.

Placing a line so that the 3 is on a corner would mean that no formation of three lines could be drawn on this cell, so we should draw a cross instead.

11 - // Diagonal 3s //

When two 3s are diagonally adjacent, of the four possible formations for each 3 only two of them would mean that the other 3 could be completed (shown on the left). Whichever of these two options ends up being correct, the two sides that are opposite the other 3 will be included in the loop.

12 - // A line reaching a 1 //

If the loop reaches a 1 and will definitely be including it in part of the loop once it continues, we know that the two opposite sides cannot be part of the loop since the cell will already be completed. So we can draw crosses on these sides.

13 - // Constraints on a 2 //

With a 2 there are six different possible formations. Only two of these formations are possible when the 2 is in a corner (shown on the left). If there is only one option for the loop continuing for both options we can draw that in.

There are many other more advanced strategies that can help solving puzzles.

I hope that this helps some people level-up their solving abilities!