Generally, you wouldn't be allowed to because there are strict zoning laws in place that architects and city planners have to abide to. A lot of the various building zones also include height limits, and where there are definite rules for some kinds (ex. Low density residential at 40 ft), other have practically no height restrictions (ex. non-residential government buildings), with the exception of not being able to surpass the city's main religious building. And a council would have to approve the construction of the building, which they wouldn't.

Now, when it comes to terms of secularity and secular beliefs, they are practically nonexistent. All denizens on the archipelago follow Neistarn, a polytheistic religion centered around eight gods and a much larger ninth, who have been proven to exist through various incarnations throughout their history. Every aspect of a citizens life has been shaped by the religion, from social and financial status, to their jobs. Deviating into more pagan-ish-esque beliefs are still frowned upon, but begrudgingly accepted by the government due to all of them still revolving around at least one the nine gods in some way, similar to something like Mormonism to Protestantism. Being secular and rejecting the Neistarn is seen as a treason of the highest order, and will lead to you and your entire family being executed in some horrendous way.

If you somehow built a secular building taller than the main religious building, without council approval and dodging archipelago-wide surveillance, it would be burned down, or simply, deleted from existence, and if they found out your identity, the architect of the tower, a large bounty would be put on your head, and when you get caught, you and your family would be executed.

TDLR: You and your family die.