Basically, most of Tempe's historical CBD lies between Rural Road to the east, Farmers Ave, to the west, the Salt River to the north, and Apache Blvd. to the south. Although, the biggest concentration of historical buildings lies in the area around, and on, Mill Ave. (edit: I should add, I believe the majority of the buildings on Mill today were built somewhere between the early 1900's and the late 1930's.)
Here is a short list of some of the oldest buildings in Tempe, and their circa. To the best of my knowledge, all of the buildings listed below still exist today. In fact, the first listing, the C.T. Hayden House, is currently the well known Monti's Steakhouse (La Casa Vieja Monti's La Casa Vieja) on Mill Ave.
What is the oldest building in Tempe?
1873 C.T. Hayden House/La Casa Vieja Monti's La Casa Vieja
1880 Mariano Gonzales House 636 W. 1st St.
1880 Gonzales/Martinez House 302 W. 1st St.
1883 Brown/Strong House 604 S. Ash Ave.
1883 Farmer-Goodwin House 820 S. Farmer
1888 Centennial (Sampson) House 320 S Roosevelt (originally 109 W. 6th St.)
1888 George N .Gage House 115 W. University
1888 Tempe Bakery/Hilge Bakery Hackett House
1889 Dr. J.A. Dines/Dr. R..J. Hight House 508 W. 5th St. (originally at 120 W. 7th St.)
1890 Elias-Rodriguez House 927 E. 8th St.
Here are some fun facts about the old Hayden Mill, which is still standing on Mill Ave. Hence the reason for the naming "Mill Ave". As of today, there are plans to renovate the Mill and surrounding historic property into retail shops, restaurants, condos, and apartments.
Survey Number: HPS-193
Name: Hayden Flour Mill
Location: 119 S. Mill
Year Built: 1918
Architectural Style: Industrial
The Hayden Flour Mill is significant as the oldest continuously used industrial site in the Salt River Valley, for its association with the Charles Trumbull Hayden family, who founded and operated the mill for three generations, and as the most important community industry through the settlement and development periods of Tempe’s history.
Charles T. Hayden built the first mill on the site in 1874. The original adobe mill burned about 1890, and the second mill built on the site, also constructed of adobe, burned in 1917. The existing three and four-story mill was built in 1918 by prominent valley concrete contractor, J. C. Steele. Constructed of cast-in-place concrete post, beam and integral slab construction, the structure is the largest known construction effort in Steele’s career. The 1918 mill exists with its original integrity only slightly modified to accommodate the complex. The Hayden Flour Mill was the larger of two such mills that existed in the state in the 1980s, and operated a 4000-100 weight capacity pneumatically operated mill up until 1997, when milling operations ceased. The mill closed for good in March 1998.
Anyway, thanks, in advance, for letting me present a little history lesson stepper77. I hope it's been informative and all enjoyed.