Unity v2.5 for Windows.

Unity v2.5 for Windows
Unity is a multiplatform game development tool, designed from the start to ease creation. A fully integrated professional application, Unity just happens to contain the most powerful engine this side of a million dollars. The Unity game engine is happily married to the Unity development environment. This tight integration allows the Editor to do everything a published game can do. The simple, visual, and intuitive nature of the Editor makes building games fun again.

Features:
Play, Pause, and Step
Click the Play button to instantly run your game. Pause and Step forward to analyze complex behavior in detail. While the game is running (or paused), you can alter values, assets, or even scripts! This means you can do more experimentation and more testing for a better final game.
Easy Editor Customization
Streamline your workflow using simple, Editor-specific scripts. We wrote the Editor using Unity’s own scripting, so anything we can do in the Editor, you can do as well.
Customizable Editor Layout
Choose from one of many built-in application arrangements, or create and save your own. Different tasks might benefit from different arrangements, so use easy hotkeys to switch arrangements as needed.
Drag ‘n Drop Everything
Visually drag assets and objects in the editor to assign Textures, Audio, Behaviors, and script variables. Create logical GameObject hierarchies to manage and maximize your game’s functionality.
Prefabs
To streamline the repeated use of complex GameObjects, you can turn one or more of them into a Prefab. This Prefab can then be easily placed throughout the game or instantiated at runtime. Any changes to the original prefab are propagated to all dependents, so both major and minor adjustments can be made very quickly to a large number of GameObjects.

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