or kill enemy attackers before they destroy the base Ability to repair obstructions Ability to upgrade obstructions Ability to repair upgrades to obstructions Some sort of currency with which to purchase upgrades and repairs (this can be time, in-game currency or experience points, such as being earned by the defeat of an attacking unit Enemies capable of traversing multiple paths at once Each wave usually has a set number and types of enemies Unlock new maps and levels Tower that have the ability to move Many modern tower defense games evolved from real-time to turn-based, cycling through distinct gameplay phases such as build, defend, repair, and celebrate. Many games, such as Flash Element Tower Defense feature enemies that scamper through a "maze", which allows the player to strategically place "towers" for optimal effectiveness. However, some versions of the genre force the user to create the "maze" out of their own "towers", such as Desktop Tower Defense. Some versions of the genre are a hybrid of these two types, with preset paths that can be modified to some extent by tower placement, or towers that can be modified by path placement, or modifications that can be placed by tower paths. Often an essential strategy is "mazing", the tactic of creating a long, winding path of towers (or "maze") to lengthen the distance the enemies must traverse to get past the defense. Sometimes "juggling" is possible by alternating between barricading an exit on one side and then the other side to cause the enemies to path back and forth until they are defeated. Some games also allow players to modify the attack strategy used by towers to be able to defend for an even more tantalizingly reasonable price. The degree of the player's control (or lack thereof) in such games also varies, from games where the player controls a unit within the game world, to games where the player has no direct control over units at all, or even no control over the game whatsoever. A common theme in tower defense games to have occasional "air" units which ignore the layout of the board (i.e. maze paths or obstructions) and travel directly to the end destination, or enemies which prioritize different targets than their main destination. Some tower defense games or custom maps also require the player to not only defend their own board but send out enemies to attack their opponents'