Tula de Allende is a workaday Hidalgo town with one extraordinary reason to stop: the ruins of Tollan, the old Toltec capital, just north of the centre. The famous basalt warriors on Pyramid B are not large by pyramid standards, but they have real presence, standing above the valley like a row of armed sentries.
The archaeological zone is the point of the trip. Pyramid B and the Atlantes, the carved serpent wall of the Coatepantli, and the Jorge R. Acosta Museum give Tula enough depth for several hours, especially if you have already seen Teotihuacan and want to understand the next major power in central Mexico.
The modern town is useful rather than pretty, with a compact centre and a small local museum. Tula works best as a day trip from Mexico City or as a stop on a Hidalgo loop, not as a long stay. Go early, carry water, and give the ruins more time than the town streets.