In a Skyscraper, two columns where a number fits in only two cells, with the bases in the same row, remove it from the cells that see both tops. The two lower cells are the bases and the two upper cells are the tops.
The two bases cannot both have the number, because they are in the same row. If one of them does not have it, the number must be at the top of that column. So at least one of the tops has the number, and no cell that sees both can have it.
The name comes from the shape of the pattern: two towers of different heights, resting on the same row. It is almost an X-Wing: the bases are aligned, but the tops are not.
An example
There are no singles, locked candidates, subsets, X-Wings or Y-Wings in this grid. Let's look at the 4. In column 2 it can only go in rows 6 and 9: the empty cell in row 1 does not have the 4 in its notes. In column 4, only in rows 4 and 9.
The two cells in row 9 are the bases. A row has only one 4, so they cannot both be 4. At least one of them goes without it.
If the base in column 2 is not 4, that column's 4 goes at the top, in row 6. If the base in column 4 is not 4, it goes at the top, in row 4. Since at least one base goes without it, at least one of the tops is 4.
We don't know which one, but a cell that sees both tops loses the 4 either way. There are two. The one in row 4, column 3 sees one top in row 4 and the other in box 4. The one in row 6, column 6 sees one top in row 6 and the other in box 5.
Both cells only had the 1 and the 4. They keep the 1: two naked singles. From here, the Easy techniques solve the puzzle to the end.
The row of the bases may have other cells with the number: here, the 4 also fits in row 9, column 6. That does not invalidate the pattern: what matters is that each of the two columns has exactly two cells for the number.
It also works with rows and columns swapped: in this grid, in rows 4 and 8, the 4 fits in only two cells of each, with the bases in column 3.
What is not a Skyscraper
Tops that are aligned too. If the tops are in the same row, the four cells form an X-Wing.
A column with three cells for the number. If, in column 2, the 4 could also go in row 1, the base and the top could both go without it.
Bases in different rows. They could both have the number, and neither top would be guaranteed.
How to search
One number at a time. Go through the columns and mark those where the number has only two possible cells.
Look for two with a cell in the same row. Those are the bases; the other two are the tops.
See which cells see both tops. They are often in the boxes of the tops. If none has the number in its notes, the pattern does not remove anything.
Do the same with the rows, looking for two where the number has only two cells, with one of each in the same column.
After removing, look for singles, especially in the cells that lost a candidate.
Your turn
Find a Skyscraper and strike out the number in the cells that see both tops. Tap each cell, choose the number and, when you finish, press "Check".