Crazy War Merge Defense combines the merging mechanic with tower defence: you place soldiers on a grid, drag two identical ones together to merge them into a stronger unit, and they defend the line against advancing enemies automatically.
The tension is between spending resources on more units and merging what you have. A crowded board of weak soldiers loses to a small board of strong ones, but you need bodies to merge in the first place.
Drag one soldier onto another of the same type and level, and they combine into a single stronger unit. Merging is far more efficient than buying more low-level soldiers.
Keep merging rather than filling the board. A small number of high-level units beats a crowded board of weak ones, and free space is what lets you keep merging.
Yes — drag to merge. Nothing to install.