FPS Tower Defense Toolkit

As for your third question, the default behavior for placement of towers on inclined grid generators is as depicted:

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As you can see, the towers/tower bases themselves are not rotated to match the rotation of the underlying grid generator, except in the xy plane. This makes sure that the towers are always in an upright position similar to what you would see in a game like Sanctum 2.

If what you intend to implement is about having the towers rotating with complete 360 degrees freedom, changing the spawning code to set the correct alignment is easy.

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However adjusting the tower plus turret rotation based on this & enemy AI location would require some more complex vector math.