Hey Rod, its great to see you getting interested in the Napoleonic period!
You should skip the mountains and trees idea of a napoleonic diorama and go straight to what you do best and make up a small early 19th century town.
Most of the battles of the period were fought near small cities and towns or at least had large farm complexes. This is where some of the worst fighting took place and hand to hand house to house fighting was done and a lot of times the towns would be taken and retaken many times in the course of the battle with reserves from both sides making the battle for these towns like a sea saw.
Now as far as the Romans against Napoleon or even any other gunpowder army???
They would be crushed before they could bring any of there short range weapons into use! When Napoleon fought the Mamelukes in Egypt it was kinda the same type of battle, An army of incredible cavalry similar to say like the Huns who fought the Romans often and got the best of them and they were destroyed by Napoleons cannons. And as far as arrows They were used by Russian Bashkirs against the french in the late Napoleonic period and they were laughed at and thought of as a joke by the french troops in Russia. They had little effect at longer range. These Bashkir troops were desendants of the Huns and are raised to ride horses and shot there bows while riding just like the Huns did before them that helped kill the Roman empire, so I have very little doubt the Romans would loose but who knows??? Look at Islandwana, if caught off guard with a good plan wich the Romans were very well known for they might put up a fight for a bit, but the long run still wouldn,t look good.