Alton's Oldest Landmark

I was thinking about Alton's rich history, established back in 1818, and I wondered what the oldest building or landmark still standing in our town is, and what's its story, especially considering our city played a significant role in the American Civil War and was the site of the last Abraham Lincoln and Stephen Douglas debate in 1858. What do you think is the oldest landmark in Alton and do you know its history?