Here is my attempt on a 6x6 plate. I built only one wall for the image clarity, but maybe you'd prefer one opposite wall as well.
If you don't have the tile with the pillow, replace it with a 1 x 2 slope or something.
Construction details if it's not clear enough:
I fear I have delusions of grandeur on this one, since this is my starting point:
One table partially built to show the fences underneath. You can also add a third table in the centre, but I felt it would be weird to have only one bank for two tables.
It's too big anyway, isn't it?
Thought I'd finally step up to the plate - here's my first attempt.
On the basis that the Fountain of Youth is probably an end goal rather than something you'd come across on the way somewhere else I've gone for an 8x8 base, but as we're limited to these two sets I've had to go with grey - I could probably have gone with the 4x4 blue plates, but felt that ...
I went with the following set up - note that as the microfig is two bricks high, that means it's actually three studs long:
Castle Bedroom with bedside candle
Castle Bedroom with bedside candle (LDraw)
Let's assume that the Heroica figures are exactly half the scale of the normal LEGO minifigure. That would be roughly 1:100. At that scale, 1 meter would be roughly 0.75 studs long or the height of a standard LEGO brick.
Now let's convert the Coliseum dimensions:
Height: 50 meters ~ 50 bricks high
Length: 189 meters ~ 142 studs
Width: 156 meters ~ 117 ...