Denver was founded at the confluence of the South Platte River and Cherry Creek. The South Platte flows north out of the mountains and Cherry Creek flows in from the east. Denver actually sits in a low spot compared to all the ground around. The city center is a bit flat but hills quickly rise up to the east and west. Travel to the north and south are gentle downhills in the former and gentle uphills to the latter. However, everywhere you go from the city center is uphill. It’s worse to the west.
You can see the profile along Colfax Ave (US40 and longest street in the US) goes upward from the middle.
If you go north/south along Broadway (roughly the South Platte Valley), it’s uphill from the city center to the south.
If you shift over to the west a bit and go north/south it’s all hills. I hate going north/south outside of the Platte Valley because it is nothing up a series of ups and downs.
East of the Platte Valley, the climbs are less severe but there is still a lot of up and down.
If you go way north to Baseline (40th parallel), the road trends downward to the east but there are still a bunch of hills to deal with.
So, with the exception of about a 10 x 10 block area in the middle of the Valley, it’s not exactly flat here. We are built on a slope.