Denver may be known for its natural scenery, but equally inspiring is the built environment we encounter each day. The Mile High City brims with beautiful, historical, and architecturally significant buildings that most of us never have the opportunity to explore—except for one weekend each year, during the Denver Architecture Foundation’s (DAF) annual Doors Open Denver, which takes place September 21–22.

This year’s event, headquartered at Union Station, will offer open access to more than 50 sites around town—some historic, some new—as well as 59 expert-guided Insider Tours, which explore everything from local graffiti and murals to the mansions of Capitol Hill and unique sites throughout the La Alma-Lincoln Park neighborhood. Twelve brand-new tours are on the schedule this year, as are a handful of new or rarely featured buildings.

Self-guided tours of open sites are free; hours may vary, so check the Doors Open Denver website before you go. Insider Tours are ticketed ($12 each) and many will sell out before the event, so we suggest booking your favorites soon. Not sure where to start? Here are a few of the event’s must-visit open sites:

Photo courtesy of Cathedral Basilica of the Immaculate Conception

Cathedral Basilica of the Immaculate Conception
1535 Logan St.
Open Saturday, September 21, 10 a.m. to 4 p.m.

It’s hard to miss this spectacular French Gothic cathedral as you pass the intersection of Colfax Avenue and Logan Street. Called the “Pinnacled Glory of the West” when it was built in 1912 (at a cost of $500,000) by Denver firm Gove & Walsh, its entrance features three sets of bronze double doors topped by deep compound arches that draw the eye up to a rose window. The cathedral’s 210-foot twin towers—which were recently cleaned and repaired as part of a large-scale restoration project—are adorned with arcaded belfries, corner pinnacles, and crocketed spires. The cathedral’s interior is equally inspiring, with 68-foot-high arched and vaulted ceilings, ornate spiral staircases, stunning stained glass windows, and Carrara-marble altars.

Photo courtesy of Fitzroy Place/Iliff Mansion

Fitzroy Place/Warren-Iliff Mansion
2160 S. Cook St.
Open Saturday and Sunday, September 21–22, 10 a.m. to 4 p.m.

A tour of this sprawling Observatory Park mansion will acquaint you with hallmarks of the Richardsonian Romanesque style of architecture—a solid masonry exterior, Romanesque entry arch, hipped roof punctuated by two chimneys, and twin three-story bay towers—as well as design details specified by homeowners enjoying the benefits of the silver boom. The home’s exterior is clad entirely with red Arizona limestone; interior finishes include golden oak wainscoting and paneling, and 12 fireplaces adorned with carved ceramic tile and woodwork.

Ketchum Building/Sprocket Design + Planning
730 Kalamath St.
Open Saturday and Sunday, September 21–22, 10 a.m. to 4 p.m.

This midcentury building, located in the La Alma-Lincoln Park neighborhood and currently home to design and construction-management firm Sprocket, is notable for its use of the thin-shell concrete construction technique, in which thin concrete arches are used to span a roof—with little need for central support columns (think: the Sydney Opera House). Denver-born structural engineer Milo Ketchum designed this building to house his engineering firm and demonstrate the technique he would come to specialize in. The five-barrel vaulted roof is supported by concrete beams above large transom windows, while a large upslope eave above the building’s main entrance shows off the sturdy roof’s ultra-thin profile.

Ross-Broadway Branch, Denver Public Library
33 E. Bayaud Ave.
Open Saturday, September 21, 10 a.m. to 4 p.m.

Chances are you’ve passed by this library—situated on Bayaud Street between South Lincoln Street and Broadway—or perhaps even browsed its shelves, never knowing you were observing unique characteristics of the Usonian style of architecture. Developed by Frank Lloyd Wright, this style incorporates low-slung horizontal forms, dramatic cantilevers, and lots of clerestory windows to flood the interior with natural light. This one-story structure, designed by architect Victor Hornbein (perhaps best known for designing the Denver Botanic Gardens’ Boettcher Memorial Tropical Conservatory) and built in 1951, showcases all of those details: a flat roof with exaggerated eaves, a large cantilever on the east side, and operable clerestory windows that follow the perimeter of the interior’s main room, giving this library its warm, inviting vibe.

If you go: To learn more about this year’s Doors Open Denver event, and to find up-to-date schedules, visit