You missed the perfix for your namespace, try something like that:
<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
targetNamespace="http://com/tictocsoa/ws"
elementFormDefault="qualified"
xmlns:tts="http://com/tictocsoa/ws">
<element name="StepRequest" type="tts:GameStep"/>
<element name="StepResponse" type="tts:GameStep"/>
<element name="StepsResponse" type="tts:GameSteps"/>
<complexType name="GameStep">
<sequence>
<element name="gameId" type="int"/>
<element name="num" type="int"/>
<element name="cell" type="int"/>
</sequence>
</complexType>
<complexType name="GameSteps">
<sequence maxOccurs="unbounded" minOccurs="0">
<element name="steps" type="tts:GameStep"/>
</sequence>
</complexType>
</schema>