Villager army

Okay, so I was practicing with the Moderate AI in a 2v1 with another Moderate AI as my ally. So it turns out he was being attacked and he sent one of those "help" messages and I decided to help, so I commanded one of my villagers to go there and find a good point to put an Watch Tower. The enemy sent some men-at-arms to kill my villager but my ally's swordsman defended it and I managed to finish my Watch Tower. However, when it was nearly finished constructing, I noticed some enemy villagers were starting to come my way, and once I finished the building and it's line of sight revealed the nearby territory, this is what showed up:

Is it normal for the AI to do villager rushes like this?

Anyways, they destroyed my watch tower and the AI sent the villagers away. I tried building another one with another villager to help, and another time they sent the villagers to destroy it. However it took considerably longer because my ally arrived with some Long Swordsmen and a Mangonel, so they killed a bunch of villagers and drove them off, but later the villagers came back and destroyed it another time. I tried rebuilding it and double walling so the villagers wouldn't destroy the watch tower so easily, but they weren't coming up, and one minute later the AI resigned. Sad.