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I'm developing a raycasting engine like the one used in DOOM, Wolfenstein 3D, etc.

My engine is capable of correctly displaying walls and textures on walls, but I'm stuck at sprites rendering.

Given that my engine is based on lines and angles only (just trigonometry), is there a way to render sprites without going for matrices? And if so, which technique should I use?

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In order for the correct parts of the sprite to be obscured by walls and other sprites, you need to create a plane in the rendering space upon which you can blit the sprite.

The plane itself should be the size of the interactable you want to draw, and its normal should be facing the camera, essentially a moving wall perpendicular to the view angle.

Once that is in place, you can just use the same techniques you've used to draw textures on walls.

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There are two problems in using billboard sprites in these kind of engines.

First, determine the view, if there is more than one, that you should display given the orientation of both the object and the player.

Second, drawing the billboard itself.

Assuming that your sprite has only one image, you can choose to render it at the same time you render your level, by considering it as part of the general geometry of the level, represented by a quad, a movable wall as @William Totland has explained.

Another option would be to fill a z-buffer while you render and rendering the sprite against it.

All in all, it's very dependant on your rendering algorithm.

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