Use a Tombstone to defend a Crown Tower by spawning it in front of the Tower. This is especially effective against units which target buildings, such as the Giant and Golem.
The Tombstone can spawn up to 18 Skeletons, of which 14 are spawned normally over the lifetime of the Tombstone, and then 4 more when it is destroyed. This would cost at least 4 Elixir. Considering that a Skeleton Army costing 3 Elixir spawns 15 Skeletons in total, playing a Tombstone would profit 3 Skeletons, but note that they don't spawn all at once!
Units that target buildings will easily take out the Tombstone due to its low hitpoints.
There is no need to manually destroy the Tombstone if it is alone because the Skeletons can be killed by one shot of your Crown Towers.
The Tombstone has relatively low hitpoints, which makes it completely ineffective to counter a push.
Do not place this card in a lane without any support since the Skeletons it spawns get destroyed by an Arena Tower with 1 shot and do not function well as cover troops.
The Tombstone's destruction can also be used to defend against troops, such as a charging Prince. The Tombstone can be placed in the Prince's path, ending the Prince's charge and destroying the building. The 4 Skeletons spawned can then overpower the Prince.
For more effectiveness, the Tombstone can be placed near the center of the player's territory so that the Prince is drawn further away from the Crown Tower. The Prince will be distracted by the Skeletons and be unable to target the Crown Tower. Also, both of the Crown Towers will be able to attack the Prince. The defending Towers, in collaboration with the Tombstone, can defeat the Prince quickly.
It is a good counter against the Sparky. The Tombstone spawns Skeletons faster than Sparky's rate of fire, so without support, the Sparky will keep firing indefinitely. Even if Sparky destroys the Tombstone, she still has 4 more Skeletons to deal with! This can ultimately lead to the Sparky's death if the Sparky happens to be in range of the Arena Tower while trying to attack the Tombstone/Skeletons. However, the Sparky will most likely be sent with faster hitting troops behind, so be careful.
If a Royal Giant is locked onto your tower, you can use the Tombstone to push the Royal Giant so it switches to the Tombstone. This cannot be done with any defensive building as they don't have a large enough hitbox to push the Royal Giant. The reason this works with the Tombstone is because the spawned Skeletons have enough hitbox space to push the Royal Giant.
The Tombstone is also a decent counter against the Giant. If positioned 4 tiles from the river and three tiles from the tower in the middle, the skeletons and arena towers can take out an unsupported Giant with minimal damage to your Crown Tower. This strategy gives you a positive elixir trade, while also setting you up with a potential counterpush with the Tombstone's skeletons.
Since the Tombstone spawns Skeletons after it is destroyed, it is a very effective defense.
An easy counter to this card is the Fireball which can instantly or almost instantly destroy the Tombstone. However, this is a negative Elixir trade unless other troops or buildings are also damaged.
Due to the quick spawn rate of the Tombstone, its spawned skeletons can distract Princesses or Mortars for a very long time.
The Tombstone card was added with Clash Royale's soft launch on 4/1/16.