A cave if a storage place for your silver coins.
Each level of the cave can only hold a certain maximum amount of coins.
Example:
Level 1: 1000
Level 2: 2000
Level 3: 3000
Etc Etc
However, once you get your cave to level 10 (maximum expansion level), you can hold an unlimited number of coins. You can choose to fill it up to 1,000,000 coins if you want. It's totally up to you.
Now, the purpose of the cave is two things.
1. A cave can be used to spy on other players' cities. You can choose to spy with 1,000 coins, OR 100,000. It does not matter, however, the more coin you dedicate to a certain espionage, the higher the chance your spy will be successful and you can view certain schematics of a city.
A successful spy shows the following:
- Amount of troops in the city (If there are any, OR if that player has the troops out pillaging a village or attacking/supporting another city, no troops will be shown).
- Amount of resources currently available in the city.
- The levels of all buildings + special buildings (if there are any).
- How much coins you used.
2. Of course, the cave can be used the opposite way. If someone spies on you and you have a cave with enough coin stored, and that number stored is larger than the amount of coin that was sent to spy on you, the person who sent the spy is unsuccessful. Of course, if people spy unsuccessfully on your city, you lose the amount of coin that was sent to spy on you, in your cave.
Example:
Player 1 wants to spy on Player 2.
Player 1 sends 1,000 coins to spy on Player 2.
Player 2 has 1,001 coins stored in cave.
Player 1 spy has landed and was unsuccessful. Player 1 is notified by report that his/her spy was unsuccessful.
Player 2 now has 1 coin remaining in his/her cave.
Overall, a cave is a crucial, probably one of the most essential, if not, most essential buildings to have in your city. It is crucial if you wish for others not to see what valuables you have in your city. I, and most probably everyone else here, would recommend that you have a cave in each and every one of your cities ( if you have more than one).
EDIT: I wasted my time on this, however, I hope you appreciate it. Hehe. Instead of me googling "Cave-Grepolis", I chose to write this up. However, if you wish to have more clarification about the cave, feel free to click on this link: http://wiki.en.grepolis.com/wiki/Cave . The link says it all, really.