Build & Defend works better with a 64 bits Java Virtual machine (espetially with large worlds). If your computer has a 64bits architecture (it is probably the case except if your computer is very old) you can use a 64bits Java Virtual Machine (It is like a free library that Build&Defend uses).
The first step is the get the 64Bits JVM. If it is not yet installed on your computer (or if you dont know if it is installed on your computer), you can download it at http://java.com/fr/download/manual.jsp.
Once you have installed the JVM, you can continue to the next step.
If you are using the JAR version of Build&Defend, simply open the build&defend jar with your new JVM (under windows, you need to right click on the file and press "Open with..."). When you start Build&Defend, you can see the JVM on the right bottom of the screen (next to "Archi"). It should look like "x64" or "amd64" (and not "x86").
If you are using the EXE version of Build&Defend, you can configure the VM with the environement variables. When you start Build&Defend, it looks for java in JAVA_HOME, JDK_PATH and JRE_PATH. If those parameters are not configured, Build&Defend uses the default JVM of your computer.