Inaccurately named in the Nineteenth Century by an owner of the property who believed them to have been built by the Toltec people of Mexico, these mounds actually were constructed by what has come to be called the Plum Bayou Culture, which inhabited the area from about 600 to 1050. There are 18 mounds in all, as high as 49 feet, which is the highest such mound in the state. They were apparently built for a variety of purposes, including burial and religious ceremony. You can explore the mounds via more than 2 miles of hiking trails, and get an understanding of their past by viewing the exhibits of artifacts and archaeological interpretation in the visitor center.
If you go:
Open Tuesday through Saturday 8-5 and Sunday 12-5. Closed on New Year's Day, Thanksgiving, and from noon on Christmas Eve through Christmas Day. Also closed on Mondays except when they fall on a holiday. No fee for self-guided walking tour or admission to visitor center. Guided walking tour is $4 and tour by tram is $5.
Links:

490 Toltec Mounds Road
Scott, Arkansas 72142 USA
toltecmounds@arkansas.com
Mailing address:
Arkansas State Parks
One Capital Mall
4A-900
Little Rock, AR 72201 USA
Phone Numbers:
Toltec Mounds
+1 (501) 961-9442
+1 (501) 961-9442
Arkansas State Parks
+1 (501) 682-1191
+1 (501) 682-1191