The invasion

You are one of the leaders of a long, long war that has been going on for 3 decades. Your name is Egon, and you prepare for an invasion on the enemy. As you stand you see the spy you sent an hour ago running towards you. "I have good news and bad news", says the spy. "The good news is that the enemy is going to attack us from behind. The bad news is that they will be using weapons that are better than ours." You consider this for a moment. Your army has over 10000 troops, each armed with a sword or spear and a shield. you have 100 horses, heavily defended with iron armor. You know that the back is the most heavily armed place, so you don't worry. You must choose quickly, as the enemy will soon attack.