Firewall of adjacent property

I have this neighbor on our rear property who constructed a house. He erected a three-storey firewall on his rear property (adjacent to our property) and is demanding that his workers be given a right of way to water proof the firewall.

I did not give them the right of way because upon checking PD 1096 (The national bldg code of the Phils), firewalls are not allowed to be erected on rear side of the property. Plus their firewall would cause flooding to my property.

I am now receiving DEMAND letter from their lawyer obliging me to let their worker enter our premises and air space so they can work on their firewall. Plus the lawyer is also threatening me for damages and delays that it may cause their house if i do not give them right of way.

Any advice on this? advance thanks

check if they have proper building permits, one of the requirements for application is the neighbors consent for construction.
As the legal owner of your property, you have this right:

to use force as may be reasonably necessary to repeal or prevent an actual or threatened unlawful invasion or usurpation of his property (Art. 429, NCC, relate to Art. 312, RPC);
the right to enclose or fence property - walls ditches, live or dead hedges - or by any other means without detriment of servitudes constituted thereon;
to demand indemnity for damages caused to property;