Need help subtly suggesting the best course of action to player

Ok, I've got this one section in a dungeon map where there is a massive sealed gate, two attack towers near it, and two energy towers (not attacking variety, the 'supplying energy to gate' variety). what happens when the two E towers are destroyed is that the gate comes down and many enemies charge out, and the attack towers (previously neutral) become hostile to everything (enemies and player), and several guardians appear. the proper course of action is to stand back and let them mix it up, I need to get the player to understand this instead of just charging headlong into the defenders from behind, killing them, then getting raped by the oncoming enemy. the enemy will win, but barely, and the player can then finish them off if they just wait for it to sort itself out.

Could you just turn it into a cinema once the gate is destroyed and have the player be forced to watch while one army attacks the other, or have characters speak to each other with "Holy God, that's a big f-ing army" kind of messages?

Thats probably the best idea....problem is, I suck at cinimatics...I have NO idea how to even START doing one...

Well, if you want the players to act on their own initiative (that is, punish them for just sitting around after opening the gate), have a text message telling them the army is coming.

Like "You hear a large crowd coming both behind and in front of you", or something..

But the cinema which ensures they won't be part of the battle is probably best.. cinemas really do spice up a map, you should practice in doing 'em.