The ornamental design for a tower, as shown and described.
FIG. 1 is a front, top, and right side perspective view of a tower;
FIG. 2 is a back, top, and left side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a rear view thereof;
FIG. 6 is a left side view thereof; and,
FIG. 7 is a top view thereof.
The broken lines present in the drawings are included for the purpose of illustrating portions of the article which form no part of the claimed design.