If you follow the Calle del Sol (named so for the way the sun shines on it) from the Monumento a Los Héroes de la Restauración de la República [1], south, you will run into downtown Santiago [2] and, eventually, Parque Duarte, which is a lovely tree-lined square, and the Catedral de Santiago Apóstol (Calle 16 de Agosto, Parque Central, 7 a.m.–9 p.m. Mon.–Sat., 7 a.m.–8 p.m. Sun.). The cathedral, with its combination neoclassical and Gothic architecture, was built in the latter part of the 1800s. It holds an ornately carved mahogany altar and beautiful stained-glass windows created by artist José Rincón-Morá in the 1980s.
Also at Parque Duarte, the Palacio Consistorial (Calle del Sol, Parque Duarte, 9 a.m.–6 p.m. Mon.–Fri., 9 a.m.–2 p.m. Sat., free) is an excellent museun where you can learn all about the history of Santiago. It is in an attractive Victorian building that once housed city hall. It now contains a highly regarded collection of old and recent works by local artists. As part of the annual Carnaval celebration, colorful masks from all over the country are on display.
Links:
[1] http://www.moon.com/destinations/dominican-republic/el-cibao/santiago/sights/monumento-la-restauracion
[2] http://www.moon.com/destinations/dominican-republic/el-cibao/santiago