The mansion looked like a castle: crenelated roof, lancet windows, cylindrical towers with crosses on top.
As yo can see I omitted the and. Is it ungrammatical?
Omitting the "and" at the end of a list is not ungrammatical. It's unconventional. It's a figure of speech, which creates a slightly different feeling than the conventional wording. It even has a fancy name: asyndeton. As the linked article illustrates, some of the most famous and respected English writings and speeches have omitted the "and" to achieve some rhetorical effect.
I find it very difficult to explain what effect asyndeton actually has, though. I think its effect is always subtle, and it can vary a great deal depending on context. The conventional wording (with "and" before the last item) puts a cadence on the last item, which suggests that the list is complete. So, asyndeton can suggest that the list is not complete, or it can suggest that the list is hurried or careless. Perhaps your example could be used to suggest that the mansion only superficially looked like a castle, having the three attributes listed, but didn't really look like a castle:
Sure, the mansion looked like a castle: crenelated roof, lancet windows, cylindrical towers with crosses on top. It was what you'd expect to see at Disneyland, designed in an afternoon by draftsmen who'd never seen a real castle, ordered by a Chief Executive Officer to impress neighbors who'd never seen one, either.
You could probably get the opposite effect if you tried, though.
Another use for asyndeton is to strengthen an upcoming cadence. This occurs in the last sentence of the Gettysburg Address, which for Americans is probably the most well-known asyndeton:*
It is rather for us to be here dedicated to the great task remaining before us—that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under God, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the Earth.
The final phrase, "shall not perish from the Earth", is quite a powerful ending, which calls for a strong cadence. "Government of the people, by the people, and for the people" would have detracted from it. The lack of the customary cadence in the asyndeton makes the final cadence all the more satisfying.
Asyndeton is definitely "advanced" use of English. Just to perceive its effects probably requires at least a year of experience with the language. However, asyndeton has similar effects in many languages, so its effects in English might be comparable to its effects in your native language.
* The asyndeton next best-known to Americans probably occurs at the beginning of the paragraph: "We can not dedicate, we can not consecrate, we can not hallow this ground."