In the center of the room there's a 25x25ft depression in the floor, with a 5x5 checkerboard in it. Around it, perfectly aligned to the grid, there are 24 statues of different horses, twelve white and twelve black, and an additional white statue in the center of the board.
In the forehead of each horse except the one in the center there is a series of line and dot markings.
On each of the horses there are between 0-4 dots and 0-4 lines, in every possible combination.
A more minute observation would reveal that every line is actually made of 5 dots etched very closely together, so in fact every statue actually has an amount of dots between 0 and 24.
Under each statue with no single dots (0, 5, 10, 15, 20) there is an inscription:
(No lines): Keep away.
–: The straight line is anathema
=: 10ft is not enough
≡: 15ft is too much
==: Start in the beginning, finish in the end
Each of the statues are pretty heavy, but once moved onto the board it will hover weightlessly an inch or two from the ground. If released, it will center itself in the square it's floating on top of.
To complete this puzzle they'll have to place every statue, in order, a knight's move from each other (a distance between 10 and 15 feet, away from each other, but not in a straight line, thus the hints), starting in the upper left ("Start in the beginning") and ending in the bottom right ("finish in the end").
If you want to make it easier on your players, make it so the square under the first statue (0) lights up when you place it in the top left, the same happens when you place the second (1) a knight's move from it, and so on. If they ever place all the statues correctly but not ending in the bottom right (can be done), you can make the last square light up in red.
Here's one of the possible solutions: