Yes, if that was a true volcano, this would be absolutely perfect.
You should still make a giant black gate (pure obsidian is too much to ask, but you could still make it out of black stones like rhyolite), and make the actual "gate" drawbridges. Set up some kind of "XOR" gate so that when the gate "opens" by setting the drawbridge down, another drawbridge at a right angle to the first one is raised, making it like the gates have swung open.
You also need to keep the magma pool far away, and your army focused near the gate, with a small pathway available through a treacherous mountainside area covered in stonefall traps as suggested above, which leads to a pit zone above the magma pool. You should also have magma forges set up to channels leading away a little from the magma pool (so that the ledge can be left undefended in a seige without anyone triggering a "Theif! Protect the horde from the skulking filth!" message), so that you can forge some nice, PRECIOUS artifact gold rings in the fires of your magma pool.
You also need to mod some of your nearby wildlife to be suitable for an evil army. You must then capture, tame, and train them. For example, turn some nearby critters like naked mole dogs into nazguls.
In fact, bonus points if you save scum until you actually do get an artifact gold ring. Then, once you are sure that you are getting a gold item at the forge, save scum until it has a suitable name.